Hydrostatic transmission drive system

ABSTRACT

An improved drive system includes a pair of hydrostatic transmissions which are driven by a common engine or prime mover and are drivingly connected with different tracks of a vehicle. The input or output speed ratios of the hydrostatic transmissions are simultaneously varied by operating a single speed control valve to port control fluid pressure to pressure responsive secondary or control motors in pump and motor actuators of the hydrostatic transmissions. A separate steering control valve is associated with each of the hydrostatic transmissions. The pump actuators each include a primary or swashplate motor which is operable to vary the effective displacement of a pump unit of the associated hydrostatic transmission. To effect operation of a swashplate motor, the associated secondary motor is operated to actuate a pilot valve from a null position and port fluid under pressure to the swashplate motor. When the swashplate motor has been operated to an extent corresponding to the extent of operation of the associated secondary motor, a floating link feedback assembly returns the pilot valve to its null position to interrupt operation of the swashplate motor. Shuttle type selector valves port control fluid pressure to the secondary motor actuators. The motor actuators are generally similar in construction to the pump actuators and each includes a secondary motor which operates a pilot valve to port fluid under pressure to a swashplate motor. Pressure regulator assemblies for the hydrostatic loops of the two hydrostatic transmissions contain pressure relief valves. The pressure relief valves have a valve member which is acted upon tending to move it in an opening direction by the fluid pressure in the hydrostatic loop of the associated transmission and is acted upon by a spring and a control pressure tending to urge the valve member in a closing direction. A master relief or pilot valve is opened when the pressure in one of the hydrostatic loops exceeds a predetermined pressure. Opening of the pilot stage relief valve reduces the force urging the relief valve member closed so that it can open and maintain the pressure at a predetermined level in the associated hydrostatic loop. Operation of a parking brake for the vehicle disengages the master relief valve and limits the pressure at which the pilot relief valve opens.

Unite States Patent Bojas et al.

Primary Examiner-Edgar W. Geoghegan Attorney, Agent, or FirmTeagno &Toddy [5 7 ABSTRACT An improved drive system includes a pair ofhydrostatic transmissions which are driven by a common engine or primemover and are drivingly connected with different tracks of a vehicle.The input or output speed ratios of the hydrostatic transmissions aresimultaneously varied by operating a single speed control valve to portcontrol fluid pressure to pressure responsive secondary or controlmotors in pump and motor actuators of the hydrostatic transmissions. Aseparate steering control valve is associated with each of thehydrostatic transmissions. The pump actuators each include a primary orswashplate motor which is operable to vary the effective displacement ofa pump unit of the associated hydrostatic transmission. To effectoperation of a swashplate motor, the associated secondary motor isoperated to actuate a pilot valve from a null position and port fluidunder pressure to the swashplate motor. When the swashplate motor hasbeen operated to an extent corresponding to the ex tent of operation ofthe associated secondary motor, a floating link feedback assemblyreturns the pilot valve to its null position to interrupt operation ofthe swashplate motor. Shuttle type selector valves port control fluidpressure to the secondary motor actuators. The motor actuators aregenerally similar in construction to the pump actuators and eachincludes a secondary motor which operates a pilot valve to port fluidunder pressure to a swashplate motor. Pressure regulator assemblies forthe hydrostatic loops of the two hydrostatic transmissions containpressure relief valves. The pressure relief valves have a valve memberwhich is acted upon tending to move it in an opening direction by thefluid pressure in the hydrostatic loop of the as sociated transmissionand is acted upon by a spring and a control pressure tending to urge thevalve member in a closing direction. A master relief or pilot valve isopened when the pressure in one of the hydrostatic loops exceeds apredetermined pressure. Opening of the pilot stage relief valve reducesthe force urging the relief valve member closed so that it can open andmaintain the pressure at a predetermined level in the associatedhydrostatic loop. Operation of a parking brake for the vehicledisengages the master relief valve and limits the pressure at which thepilot relief valve opens.

PATENTED 51974 SHEET 3 BF 6 m GE PATENTEB MAR 51974 SHEET Q UF 6 FROMSTEER/NG CONTROL 2 WM 1 RING new. (4441/! new AND

FROM

CHARGE PUMP FIG-4 1 HYDROSTATIC TRANSMISSION DRIVE SYSTEM BACKGROUND OFTHE INVENTION This invention relates generally to a hydrostatictransmission drive system.

Hydrostatic transmissions are commonly used to vary the drive ratiobetween an engine or other prime mover and the tracks or wheels of avehicle. These hydrostatic transmissions may include variabledisplacement pump and motor units with a control system which isactuated by an operator to selectively vary the input to output speedratio of the hydrostatic transmission. One known control system forvarying the input to output speed ratio of a hydrostatic transmission isdisclosed in U.S. Pat. No. 3,543,515 and includes a valve assembly whichvaries the pressure of fluid conducted to control motors to effect avariation in the displacement of pump and motor units of a hydrostatictransmission. Although the drive system disclosed in this patentincludes only a single hydrostatic transmission, a pair of hydrostatictransmissions have been utilized in vehicles having two tracks so thatone transmission can be associated with each of the tracks. Drivesystems including a pair of hydrostatic transmissions are disclosed inU.S. Pat. No. 3,528,243 and in Keith et al. application Ser. No.158,455, filed June 30, 1971, for Hydrostatic Transmission Speed andSteering Control System now U.S. Pat. No. 3,727,402.

SUMMARY OF THE PRESENT INVENTION The present invention relates toapparatus for use in connection with a hydrostatic transmission drivesystem. Although certain features of the invention relate specificallyto drive systems which include a pair of hydrostatic transmissions,other features of the invention can be utilized in drive systems whichinclude only a single hydrostatic transmission. More specifically, thepresent invention includes an improved actuator assembly for varying thedisplacement of a pump unit of a hydrostatic transmission in response toa variation in control fluid pressure transmitted to the actuatorassembly from a pressure regulator type control valve. To enable theinput to output speed ratio of the hydrostatic transmission to be variedthroughout a relatively large range, a selector valve ports the controlfluid to a motor actuator assembly which is operable to effect avariation in the displacement of a motor unit of the hydrostatictransmission.

When the present invention is associated with a pair of hydrostatictransmissions, the operating speed of one of the hydrostatictransmissions can be varied relative to the operating speed of the otherhydrostatic transmission by actuating an associated steering valve. The

hydrostatic loops of the two transmissions are advantageously providedwith high pressure relief valves which limit the fluidpressure in atleast one of the hydrostatic loops in response to opening ofa commonmaster relief valve.

Accordingly, it is an object of this invention to provide a new andimproved apparatus for controlling the operation of a hydrostatictransmission wherein the apparatus includes a pump actuator assembly forchanging the displacement of a pump unit of the hydrostatic transmissionin response to a variation in the control fluid pressure and a motoractuator assembly for changing the displacement of a motor unit of ahydrostatic transmission in response to a change in control fluidpressure, at least one of the actuator assemblies including a swashplatemotor for moving the swashplate of the associated unit of thehydrostatic transmission, a pilot valve for porting fluid to theswashplate motor and a pressure responsive control or secondary motorfor operating the pilot valve. I

Another object of this invention is to provide a new and improvedapparatus for controlling the operation of a plurality of hydrostatictransmissions having variable displacement pump and motor units andwherein the apparatus includes a pair of pump actuator assemblies eachof which is associated with a pump unit of one of the hydrostatictransmissions and is operable to vary the displacement of the associatedpump unit in response to a change in the control fluid pressure, a pairof motor actuator assemblies each of which is associated with a motorunit of one of the hydrostatic transmissions is operable to vary thedisplacement of the associated motor unit in response to a change incontrol fluid pressure, a pressure regulator valve for varying thecontrol fluid pressure, a first steering or secondary valve for varyingthe control fluid pressure conducted from the pressure regulator valveto the pump and motor actuator assemblies of the first hydrostatictransmission, and a second steering or secondary valve for varying thefluid pressure conducted from the pressure regulator valve to the pumpand motor actuator assemblies associated with the second hydrostatictransmission.

Another object of this invention is to provide a new and improvedhydrostatic transmission having a relief valve which is operable from aclosed condition to an open condition to limit fluid pressure in thehydrostatic loop of the transmission, means for applying a biasing forceurging the main relief valve toward the closed condition against a forcegenerated by transmission system pressure, fluid control means normallypermitting said biasing means to hold said relief valve in said closedcondition, and pilot relief valve means responsive to a predeterminedhydrostatic system pressure and operating on said fluid control means atsaid predetermined system pressure to permit said main relief valve tomove to said open condition under the influence of system pressure.

Another object of this invention is to provide a new and improvedapparatus for use in association with a pair of hydrostatictransmissions and wherein the apparatus includes a first relief valveoperable from a closed condition to an open condition to limit the fluidpressure in the hydrostatic loop of the first hydrostatic transmission,a second relief valve operable from a closed condition to an opencondition to limit the fluid pressure in the hydrostatic loop of asecond hydrostatic transmission, and a pilot relief valve control meansfor reducing the closing force applied against at least one of therelief valves in response to the presence of a predetermined openingpressure against one of the relief valves.

Another object of this invention is to provide a new and improvedapparatus, in accordance with the next preceding paragraph, and whereinan apparatus is provided for lowering the hydrostatic loop pressure atwhich the relief valves are operated from the closed condition to theopen condition in response to operation of a brake unit to an engagedcondition.

DESCRIPTION OF THE DRAWINGS The foregoing and other objects and featuresof the invention will become more apparent upon a consideration of thefollowing description taken in connection with the accompanying drawingswherein:

FIG. 1 is a schematic illustration of a tracked vehicle having ahydrostatic transmission drive system constructed in accordance with thepresent invention;

FIG. 2 is a schematic illustration of a hydrostatic transmissionutilized to drive one of the tracks of the vehicle of FIG. 1;

FIG. 3 is an enlarged sectional view-of a pump actuator assembly foreffecting a change in the displacement of a pump unit of the hydrostatictransmission of FIG. 2, the pump unit being shown in a neutralcondition;

FIG. 4 is a schematic illustration illustrating the relationship betweenthe pump actuator assembly and a motor actuator assembly for changingthe displacement of a motor unit of the hydrostatic transmission, thepump unit being shown in a fully actuated or maximum displacementcondition corresponding to forward operation of the hydrostatictransmission;

FIG. 5 is a fragmentary sectional view of a selector valve for directingcontrol fluid pressure to the motor actuator assembly, the selectorvalve being shown in a position corresponding to reverse operation ofthe associated hydrostatic transmission;

FIG. 6 is an enlarged sectional view depicting the relationship betweena pressure or speed control valve and a secondary or steering controlvalve which are connected in fluid communication with the pump and motoractuator assemblies of FIG. 4; and

FIG. 7 is an enlarged schematic sectional view illustrating therelationship between main system pressure regulator assemblies for apair of hydrostatic transmissions and a master relief valve.

DESCRIPTION OF ONE PREFERRED EMBODIMENT General Description A vehicle 10having a hydrostatic transmission drive system 12 constructed inaccordance with the present invention is illustrated in FIG. I. Thehydrostatic transmission drive system 12 includes left and right handhydrostatic transmissions I4 and 16 which transmit drive forces from acommon engine or prime mover 20 to left and right tracks 22 and 24 ofthe vehicle 10. The hydrostatic transmissions 14 and 16 include variabledisplacement pump units 28 and 30 which are driven by the engine 20 totransmit fluid under pressure to variable displacement motor units 32and 34 which are drivingly connected with the tracks 22 and 24 bydriveshafts 36 and 38.

When the vehicle 10 is being driven along a straight path, the input andoutput speed ratios of the hydrostatic transmissions 14 and 16 aresimultaneously changed by equal amounts upon operation of a speedcontrol or pressure regulator valve 42 which is connected with thehydrostatic transmissions through steering control valves 46 and 48.Assuming that the vehicle 10 is moving forwardly along a straight pathat a relatively slow speed, the speed control valve 42 can be operatedto increase the control fluid pressure transmitted through the steeringvalves 46 and 48 to pressure responsive pump actuator assemblies 50 and52. The increase in control fluid pressure activates the pump actuatorassemblies 50 and 52 to increase the effective displacement of the pumpunits 28 and 30 by equal amounts and thereby increase the forward speedat which the tracks 22 and 24 are driven. If the speed control valve 42is operated to a sufficient extent, motor actuator assemblies 56 and 58will decrease the effective displacement of the associated motor units32 and 34 by equal amounts to further increase the speed at which thetracks 22 and 24 are driven.

If it is desired to turn the vehicle It) toward either the left or theright, one of the steering control valves 46 or 48 is actuated to reducethe output speed of the hydrostatic transmission 14 or 16 connected withthe track 22 or 24 on the side toward which the vehicle is to turn.Thus, if the vehicle is to be turned toward the left, the steeringcontrol valve 46 is actuated to reduce the output speed of thehydrostatic transmission 14 and the speed of movement of the track 22relative to the hydrostatic transmission 16 and track 24. Similarly, ifthe vehicle is to be turned toward the right, the steering control valve48 is actuated to reduce the output speed of the hydrostatictransmission 16 and the speed of the track 24.

Control fluid is supplied to the speed control valve 42 from an enginedriven charge pump 62 (FIG. 1) through an antistall system 64. In theevent of an impending overloading of the engine 20, the anti-stallsystem 64 effects a reduction in the pressure of the control fluidconducted to the speed control valve 42. This results in a reduction inthe pressure of the control fluid ported to the pump and motor actuatorassemblies 50, 52, 56 and 58 of the hydrostatic transmissions l4 and 16to effect a corresponding reduction in the output speeds of thehydrostatic transmissions. Although many different types of anti-stallsystems could be utilized, it is preferred to use the anti-stall systemdisclosed in application Ser. No. 237,086 filed Mar. 22, I972 and byEdward .I. Bojas entitled Hydrostatic Transmission Control System. V

The pump unit 28 of the left hand transmission 14 is of the axial pistontype (see FIGS. 2 and 3) and has a swashplate which is moved by theactuator assembly 50 to vary the output of the pump unit in a knownmanner. The pump unit includes a rotary barrel 74 fixedly connected toan engine driven drive shaft 76. When the swashplate 70 is pivoted in acounterclockwise direc tion from the neutral position illustrated inFIG. 3 to the forward actuated position shown in FIG. 2, high pressurefluid flows through a conduit of the hydrostatic loop 82 (FIG. 2) to themotor unit 32. Low pressure fluid flows from the motor 32 through acondiut 86 of the hydrostatic loop 82 to the pump unit 28. The rate atwhich fluid flows through the high pressure conduit 80 is increased asthe swashplate 70 is moved further from the neutral position of FIG. 3toward the fully actuated position of FIG. 2.

When the swashplate 70 of the pump unit 28 reaches the fully actuatedposition of FIG. 2, the rotational speed of the output shaft 36 isincreased, while the engine operating speed remains constant bydecreasing the displacement of the motor unit 32. To effect a decreasein the displacement of the motor unit 32, a swashplate 90 of the motorunit 32 is pivoted in a counterclockwise direction (as viewed in FIG. 2)by the motor actuator assembly 56 until the swashplate approaches, butdoes not reach, a vertical position (as viewed in FIG. 2). This movementof the swashplate 90 decreases the effective displacement of the motorunit 32 so that a barrel or body 94 to which the output shaft 36 isfixedly connected is rotated through a complete revolution under theinfluence ofa relatively small volume of fluid conducted from the pumpunit 28 through the conduit 80 to the motor unit 32.

The direction of rotation of the output shaft 36 of the motor unit 32can be reversed, without reversing the direction in which the engine 12drives the input shaft 76 to the pump unit 28, by pivoting theswashplate 70 of the pump unit 28 in a clockwise direction from theneutral position illustrated in FIG. 3. When the swashplate 70 ispivoted in a clockwise direction from the neutral position of FIG. 3,the pump unit 28 forces fluid under pressure through the conduit 86 tothereby reverse the direction of operation of the motor unit 32. Ofcourse during reverse operation of the pump unit 28, the conduit 80conducts low pressure fluid from the motor unit 32 back to the pump unit28. During operation of the pump unit 28 in either the forward or thereverse direction, make up fluid is supplied from the engine drivencharge pump 62 to the low pressure conduit 80 or 86 of the hydrostaticloop 82 through check valves 98 and 100 (FIG. 2). The check valves 98and 100 are connected with the charge pump through conduits 102, I04,106 and 108.

The right hand hydrostatic transmission 16 (FIG. 1) is of the sameconstruction as the left hand hydrostatic transmission 14 and cooperateswith the speed control valve 42 and a steering control valve in the samemanner as does the left hand hydrostatic transmission. Thus, the pumpand motor actuator assemblies 52 and 58 are activated in response toactuation of the speed control valve 42 to move pump and motorswashplates in the same manner as in which the swashplates 70 and 90 aremoved by the pump and motor actuator assemblies 50 and 56.

Pump Actuator Assembly The pump actuator assembly 50 moves theswashplate 70 of the pump unit 28 in response to changes in controlfluid pressure conducted through conduits 114 and 116 (FIG. 2) to thesteering control valve 46 and from the steering control valve 46 throughconduits 118 and 120 to a pressure responsive control or secondary motor126 in the pump actuator assembly. Operation ofthe control motor 126actuates a pilot valve 128 to port fluid from the charge pump 62 to aswashplate or primary motor 132. A floating link type feedback assembly134 interconnects the swashplate 70, pilot valve 128, and control motor126 and closes the pilot valve when the swashplate motor 132 has movedthe swashplate through a distance corresponding to the extent ofoperation of the control motor.

When speed control valve 42 is in its neutral condition, the swashplate70 of the pump unit 28 is moved to the neutral position of FIG. 3 bybiasing springs 135 in the swashplate motor 132. The pump unit 28 willthen have zero effective displacement and rotation of the input shaft 76by the engine or prime mover 20 is ineffective to cause the transmittalof high pressure fluid from the pump unit to the motor unit 32 so thatthe left-hand track 22 is not driven.

When it is desired to move the vehicle forwardly, the speed controlvalve 42 is operated in a forward direction by moving a control handle138 from the neutral position shown in dashed lines in FIGS. 2 and 6 toa forward actuated position shown in solid lines in FIGS. 2 and 6. Thismovement of the control handle 138 ports high pressure control fluidthrough the conduit II4 to the steering valve 46 and from the steeringvalve 46 through the conduit to a pressure chamber 142 (FIG. 2) of thesecondary or control motor 126. The high pressure control fluid causes apiston 144 to move downwardly (as viewed in FIG. 3) against theinfluence of a spring assembly 148 from the neutral position of FIG. 3toward the forward actuated position shown in FIG. 4. During thismovement of the piston 144, a piston rod 150 pivots a link 152 of thefeedback assembly 134 about a connection 154 to move a valve spool 158from a null position downwardly (as viewed in FIG. 3) in a valve chamber160. The valve spool 158 is constructed so that when in its nullposition fluid leakage is directed to the cylinders 168, 170 andcorresponding leakage occurs through the cylinders. Movement of thevalve spool from the null position ports fluid through a conduit 164 toone cylinder 168 of the swashplate motor 132 and connects anothercylinder 170 of the swashplate motor with drain through a conduit 174(see FIG. 4) and opening 176 leading to a sump connected housing (notshown). Of course, the fluid pressure in the cylinder 168 effectsoperation of the swashplate motor 132 to pivot the swashplate 70 in acounterclockwise direction to effect operation of the hydrostatictransmission 14 in a forward direction.

Operation of the swashplate motor 132 to pivot the swashplate 70 to aforward actuated position (FIG. 4) corresponding to the extent ofoperation of the speed control valve 42 and control motor 126 causes thefeedback link 152 to return the valve spool 158 to its null position.Thus, when the valve spool 158 is moved downwardly as viewed in FIGv 3high pressure fluid flows from the charge pump 62 through the conduit108 to the valve chamber 160 and from the valve chamber through aconduit 164 to the motor cylinder 168. This high pressure fluid forces apiston 184 in the cylinder 168 to move upwardly against the influence ofthe biasing spring in the opposite motor cylinder 170. Of course, upwardmovement of the piston 184 results in a counterclockwise pivotingmovement of the swashplate 70 (as viewed in FIGS. 3 and 4) and downwardmovement of the opposite piston 192. As the swashplate 70 is pivoted, alink 196 which is pivotally connected with the swashplate at 198 pivotsthe link 152 about a connection 202 to the secondary motor piston rod tomove the valve spool 158 from the open position of FIG. 4 toward theclosed position of FIG. 3. The general mode of interaction between thefeedback linkage 134, secondary motor 126, pilot valve 128 andswashplate motor-132 is, in certain respects at least, the same as isdisclosed in US. Pat. No. 2,396,448.

When the swashplate 70 has been moved by the swashplate motor 132 to anextent corresponding to the extent of operation of the secondary orcontrol motor 126, the feedback linkage 134 operates the pilot valve 128to its null condition to interrupt further operthe cylinders I68, 170),the link 134 would be operated to move the pilot valve spool 158 andimmediately compensate therefor.

Assuming that the operating speed of the engine is not changed, changingthe angular position of the swashplate 70 results in a correspondingchange in the rate at which fluid is pumped under pressure through theconduit 80 to the motor unit 32 and the speed at which the shaft '36 isrotated in a forward direction. Due to the effect of the feedbacklinkage 134, the extent to which the swashplate 70 is moved by theswashplate motor 132 is related to the extent to which the secondary orcontrol motor 126 is operated by a change in pressure conducted from thespeed control valve 42. However, the change in pressure conducted fromthe speed control valve 42 to the control motor 126 is related to thedistance which the control handle 138 (see FIG. 2) is moved by theoperator. Therefore, the operating speed of the output shaft 36 of themotor unit 32 and the speed of the left-hand track 22 is varied to anextent which is a function of the extent of movement of the controlhandle 138 when the engine 20 is operating at a constant speed.

Motor Actuator Assembly If the control handle 138 is moved through arelatively large distance in the forward direction from the neutralposition, a relatively large change is made in the output speed of thehydrostatic transmission 14. To ef fect this relatively large change itmay be necessary to increase the rotational speed of the output shaft 36in a forward direction to a greater extent than can be accomplished bymoving only the swashplate 70 of the pump unit 28. Therefore, after theswashplate 70 of the motor unit 28 has moved to the fully actuatedposition of FIG. 2, the swashplate 90 of the motor unit 32 is pivoted ina counterclockwise direction (as viewed in FIG. 4) by the motor actuatorassembly 56 to decrease the effective displacement of the motor unit.Decreasing the effective displacement of the motor unit 32 results in acorresponding increase in the output speed of the hydrostatictransmission 14.

The motor actuator assembly 56 includes a single action secondary orcontrol motor 212 which is operated to actuate a pilot valve 216 fromits null position (in which it operates as valve 128) to port fluid fromthe charge pump 62 to a swashplate motor 218. Operation of theswashplate motor 218 pivots the swashplate 90 in a counterclockwisedirection from the maximum displacement position of FIG. 4 toward aminimum displacement position. As the swashplate 90 is moved, a feedbacklinkage 222 operates the pilot valve 216 back toward it null position.When the swashplate motor 218 has been operated to movethe swashplate 90to an extent which is related to the extent of operation of thesecondary or control motor 212, the feedback linkage 222 returns thepilot valve 216 to its null position to interrupt movement of theswashplate.

The secondary motor 212 in the motor actuator assembly 56 is biasedtoward the unactuated position of FIG. 4 by a relatively strong springassembly 230 so that the motor actuator assembly 56 remains in theunactuated position of FIG. 4 until the pump actuator aslarge distancefrom the neutral position, the pressure conducted through the conduitoperates the secondary motor 126 in the pump actuator assembly 50 to thefully actuated position of FIG. 4 against the influence of therelatively weak biasing'spring 148. The control fluid pressure in thefluid conduit 120 is ported by a selector valve 232 through a conduit234 leading to a pressure or motor chamber 238 in the control motor 212of the motoractuator assembly 56. Since the biasing spring 230 isrelatively strong, the control motor 212 remains inactive until afterthe control motor 126 in the pump actuator assembly 50 reaches the fullyactuated condition of FIG. 4.

When motor 126 in the pump actuator assembly 28 reaches the fullyactivated condition, the fluid pressure in chamber 238 of the motoractuator assembly 56 initiates movement of a piston 242 against theinfluence of the biasing spring assembly 230 to actuate the feedbacklinkage 222 and operate the pilot valve 216 from its null position (inwhich it operates as valve 128) to port fluid to the swashplate motor218. As the piston 242 moves toward the right (as viewed in FIG. 4) alink 246 is pivoted in a clockwise direction about a connection 247 tomove a valve spool 252 from its null position to an actuated position.When the valve spool 252 is in the actuated position, fluid suppliedfrom the charge pump 62 through the conduit 104 is ported from a centralvalve chamber 260 around an annular land 262 to a conduit 266 leading toone power cylinder 268 of the swashplate motor 218. Simultaneouslytherewith, a second power cylinder 270 of swashplate motor 218 iscommunicated with drain through a conduit 272, around a land 274 on thevalve spool 252 and through a passage 276.

As the swashplate motor 218 is operated, the floating link 246 ispivoted about a connection 280 (FIG. 4) to a piston rod 284 of thesecondary motor 212 to move the valve spool 252 toward its null positionin which only fluid leakage flows through the conduits 266 and 272 whichleaks through the cylinders 268, 270. The feedback linkage 222 moves thevalve spool 252 to its closed position when the swashplate motor 218 hasbeen actuated to an extent corresponding to the extent to which thesecondary motor 212 is actuated against the influence of the biasingspring 230. Of course, the extent of operation of the secondary motor212 varies as a direct function of variations in the fluid pressure inthe conduit 234.

It should be noted that when the valve spool 252 is v in its nullposition, fluid leaks past the lands thereof to the swashplate cylinders268, 270 and corresponding leakage occurs through the cylinders 268, 270so that there is no tendency for a change in the swashplate position. Itshould be noted that when the secondary motor 212 is in the unactuatedor normal position of FIG. 4, the valve spool 252 is in an actuatedposition connecting the conduit 266 with drain and the conduit 272 withfluid under pressure to thereby maintain the motor unit 32 in its fullyoperated or maximum displacement position until sufiicient fluidpressure is built up in the chamber 238 to actuate the motor 212 againstthe influence of the biasing spring-230.

When the control handle 138 for the speed control valve 42 (see FIG. 2)is moved back to its initial or neutral position, the motor actuatorassembly 56 responds to the resulting decrease in fluid pressure in theconduit 234 to return the motor unit 32 to its full displacementcondition shown in FIG. 4. Thereafter, the pump actuator assembly 50deswashes the pump unit 28 by moving the swashplate 70 back toward theneutral or minimum displacement position shown in FIG. 3. Thus, theeffective displacement of the motor unit 32 is increased prior to adecrease in the displacement of the pump unit 28 upon operation of thespeed control valve 42 to reduce the control fluid pressure ported tothe pump actuator assembly 50 and motor actuator assembly 56.

When the control handle 138 is moved toward the neutral position, thepressure in the conduit 120 is reduced. This reduction in fluid pressureis communicated directly to the secondary motor 126 in the pump actuatorassembly 50 and ported by the selector valve 232 to the conduit 234leading to the secondary motor 212 in the motor actuator assembly 56.The reduction of pressure in the conduit 234 results in a reduction inthe pressure in chamber 238. The spring unit 230 can then move thepiston 242 toward the left (as viewed in FIG. 4) to thereby swing thefeedback link 246 in a counterclockwise directionabout the pivotconnection 248 and move the pilot valve spool 252 to the open positionillustrated in FIG. 4. When the pilot valve 216 is in the position ofFIG. 4, it connects the power cylinder 268 with drain and the powercylinder 270 with a source of fluid under pressure to thereby pivot themotor swashplate 90 in a clockwise direction from a relatively lowdisplacement position toward the maximum displacement position shown inFIG. 4. When the swashplate 90 has reached the maximum displacementposition of FIG. 4, the pressure in the conduit 120 will have beenreduced.

When the fluid pressure in conduit I and the chamber 142 of thesecondary motor 126 of the pump actuator assembly 50 is sufficientlyreduced, the spring unit 148 moves the piston 144 upwardly (as viewed inFIG. 4) to pivot the link 152 of the feedback linage 134 in acounterclockwise direction. This movement of the link 152 moves thespool 158 to connect the conduit 174 in fluid communication with theconduit 108 to port fluid to power cylinder 170 of the swashplate motor132. The power cylinder 168 of the swashplate motor 132 is connectedwith drain through the conduit 164 around the annular land 178 of thevalve spool 158 (see FIGv 3)]This results in the swashplate 70 beingpivoted in a clockwise direction from the fully actuated position ofFIG. 4 toward the neutral position of FIG. 3. When the swashplate 70reaches the neutral position of FIG. 3, the valve spool 158 will be inits null position.

When the control lever 138 for the speed control valve 42 is moved in areverse direction from the neutral position, that is in a clockwisedirection from the dashed line position of FiG. 2, the conduit 114 isconnected with drain and control fluid pressure is conducted to the pumpactuator assembly 50 through the conduits 116 and 118. This results in apiston 292 in the double-acting secondary motor 126 being moved upwardly(as viewed in FIG. 3) to swing the feedback link 152 in acounterclockwise direction about the pivot connection 154. Movement ofthe feedback line 152' 108 from the charge pump 62. This effectsoperation of the swashplate motor 132 to swing the swashplate in aclockwise direction from the illustrated neutral position. High pressurefluid is then conducted through the conduit 86 to the motor unit 32 todrive the motor 32 in the reverse direction Of course, the farther theswashplate 70' is pivoted in the clockwise direction from theillustrated neutral position of FIG. 3, the faster the motor unit 32 isdriven.

When the swashplate 70 reaches its maximum reverse displacementposition, the motor actuator assembly 56 isactuated to reduce theeffective displacement of the motor unit 32. Since the swashplate 90 ofthe motor unit 32 is always pivoted in a counterclockwise direction fromthe fully swashed position illustrated in FIG. 4, the secondary motor212 in the motor actuator assembly 56 is a single-acting motor which isoperated in the same direction whether the output speed of the motorunit 32 is to be increased in the forward or the reverse direction.Thus, the increase in fluid pressure in the conduit 118 is ported by theselector valve 232 to the conduit 234 to activate the motor actuatorassembly 56 in the manner previously explained.

Selector Valve The selector valve 232 ports control fluid pressure fromthe conduit 120 to the motor actuator assembly 56 during forwardoperation of the hydrostatic transmission 14. During reverse operationof the hydrostatic transmission 14, the selector valve 232 ports controlfluid pressure from the conduit 118 to the motor actuator assembly 56.The selector valve 232 includes a shuttle valve member 300 which isdisposed in a housing 306 having an upper end portion 307 (FIGS. 3 and5) which is connected in fluid communication with the conduit 120 and alower end portion 308 which is connected in fluid communication with theconduit 118.

The shuttle valve member 300 is moved between the forward and reverseactivated positions of FIGS. 3 and 5 in response to operation of thespeed control valve 42 between forward and reverse operated conditions.Dur-- ing forward of the hydrostatic transmission 14, control fluidpressure in the conduit 120 is communicated to the upper end portion 307of the housing 306 and urges the shuttle valve member 300 to the forwardactuated position of FIG. 3. Movement of the valve member 300 to thisposition is facilitated by the fact that the conduit 118 and the lowerend portion of the housing 306 are connected with drain through thespeed control valve 42. Similarly, when the hydrostatic transmission 14is operated in the reverse direction, the speed control valve 42connects the conduit 118 with control fluid pressure and connects theconduit 120 with drain. This results in the shuttle valve 300 beingmoved to the reverse actuated position of FIG. 5.

The shuttle valve member 300 ports the conduit 234 leading to thepressure chamber 238 in the motor actu ator assembly 56 (see FIG. 4) tocontrol fluid pressure upon actuation of the speed control valve 42 toeither a forward or a reverse actuated position. Thus, when the speedcontrol valve 42 is operated and the valve spool 300 is moved to theposition shown in FIG. 3 under the influence of control fluid pressurein the upper end portion 307 of the housing 306, the conduit 234 isconnected with control fluid pressure-in the conduit 120 by an annularrecess 310 disposed between annular lands 312 and 314 on the valvemember 300 and a passage 316 extending in an axial direction through thevalve member to the upper end portion 307 of the housing 306. When thevalve member 300 is in the reverse actuated position of FIG. 5, controlfluid pressure is conducted from the conduit 118 to the conduit 234through a passage 320 in the valve member and an annular recess 324formed between lands 314 and 326 on the valve member.

In addition to connecting the pressure chamber 238 with control fluidpressure during either forward or reverse operation of the hydrostatictransmission 14, the selector valve 232 connects a chamber 330 in thesecondary motor 212 (see FIG. 4) with drain during either forward orreverse operation of the hydrostatic transmission. Thus when theselector valve 232 is in the forward actuated position of FIG. 3 and theconduit 118 is connected with drain through the speed control valve 42,the conduit 118 is connected by the selector valve 232 with a conduit332 leading to the chamber 330 in the secondary motor 212 of the motoractuator assembly 56. The conduit 332 is connected with the conduit 118by the passage 320 in the valve member 300 and an annular recess 334disposed between lands 312 and 336 on the valve member.

Similarly, when the valve member 300 is in the reverse actuated positionof FIG. 5, the low pressure conduit 120 is connected with the conduit332 leading to the chamber 330 in the secondary motor 212. Thus theconduit 120, which is connected with drain through the speed controlvalve 42 during reverse operation of the hydrostatic transmission 14, isconnected with the conduit 332 through the passage 316 and the annularrecess 310. Therefore, the selector valve 232 connects the pressurechamber 238 of the secondary motor 212 in the motor actuator assembly 56with control fluid pressure and connects the opposite chamber 330 withdrain during either forward or reverse operation of the hydrostatictransmission 14.

Although only the pump actuator assembly 50 and motor actuator assembly56 for the pump unit 28 and motor unit 32 have been shown in FIGS. 2through 4, it should be understood that the pump actuator assembly 52for the pump unit 30 and the motor actuator as sembly 58 are the sameconstruction. Thus, the motor actuator assembly 52 includes a secondarymotor which is connected with the speed control valve 42 through thesteering valve 48 in the same manner as in which the secondary motor 126of the pump actuator assembly 5015 connected with the speed controlvalve 42 through the steering valve 46. The pump actuator assembly 52,like the pump actuator assembly 50, includes a pilot valve which isoperated by an associated secondary or control motor to effect operationof a swashplate motor and therby vary the displacement of the pump unit30. A feedback linkage, similar to the feedback linkage 134, is providedbetween the swashplate-of the pump unit 30 and the associated pilotvalve and secondary motor.

A selector valve, similar to the selector valve 232, directs controlfluid pressure to a single action secondary motor in the motor actuatorassembly 58 during either forward or reverse operation of thehydrostatic transmission 16 in the same manner as in which the selectorvalve 232 ports fluid control fluid pressure to the motor actuatorassembly 56 of the hydrostatic transmission 14. In addition to asecondary motor, the motor actuator assembly 58 includes a pilot valvewhich is opearated by the secondary motor to port fluid to a swashplatemotor, similar to the swashplate motor 218, to vary the effectivedisplacement of the motor unit 34. In view of their similarity ofconstruction and mode of operation, it is believed that a furtherdescription of the pump and motor actuator assemblies for thehydrostatic transmission 16 will not have to be set forth herein in viewof the foregoing description of the pump and motor actuator assemblies50 and 56 for the hydrostatic transmission 14.

Speed Control Valve The speed control valve 42 is connected in fluidcommunication, through the steering control valves 46 and 48, with thepump and motor actuator assemblies 50 and 56 for the hydrostatictransmission 14 and with the pump and motor actuator assemblies 52 and58 for the hydrostatic transmission 16. The conduits 114 and l 16 fromthe speed control valve 42 are connected through the steering controlvalves 46 and 48 with the pump and motor actuator assemblies for both ofthe hydrostatic transmissions 14 and 16. Operation of the speed controlvalve in either a forward or reverse direction from the neutral position(illustrated in dashed lines in FIG. 6) effects operation of the pumpand actuator assemblies 50 and 52 for the pump units 28 and 30 to thesame extent. If the control handle 138 is moved through a sufficientdistance to effect an operation of both pump units 28 and 30 to theirfully swashed or maximum displacement conditions, pressure responsivemotors in the motor actuator assemblies 56 and 58 simultaneously effectequal decreases in the displacements of the motor units 32 and 34.

The speed control valve 42 is of the pressure regulator type and portscontrol fluid pressure through the conduit 114 to the steering controlvalves 46 and 48 upon movement of the control handle 138 in the forwarddirection from the neutral position. When the fluid pressure in theconduit 114 has been increased to I an extent which corresponds to theextent of movement of the control handle 138 from the neutral position,the control valve 42 blocks fluid flow to the conduit 114. Similarly,when control handle 138 is moved in a reverse direction from the neutralposition, that is in a clockwise direction from the position illustratedin dashed lines in FIG. 6, control fluid pressure is ported to theconduit 116 which is connected to the steering control valves 46 and 48.

The speed control valve 42 includes a valve spool 350 which is slidablydisposed in a generally cylindrical valve chamber 352 in a housing 354.The valve cham' her 352 is connected in continuous fluid communicationwith the charge pump 62 and antistall system 64 by a fluid conduit 356.The valve chamber 352 is also connected with drain by a fluid conduit358.

Movement of the control handle 138 from the neutral position(illustrated in dashed lines in FIG. 6) to a forward actuated position(illustrated in solid lines in FIG. 6) moves the valve spool 350 from acentered or closed position within the housing 354 toward the left tothe actuated or open position illustrated in FIG. 6. This movement ofthe valve spool 350 ports control fluid pressure from the conduit 356 tothe conduit 114 and the steering control valves 46 and 48. In theabsence of actuation of one of the steering control valves 46 or 48, thefluid pressure is conducted from the steering control valves to the pumpactuator assemblies 50 and 52 and motor actuator assemblies 56 and 58.Thus, high pressure fluid from the charge pump 62 and antistall system64 entersthe valve chamber 352 and passes through an annular passagebetween two circular. lands 362 and 364 on the open valve spool 350 tothe conduit 114. This high pressure control fluid also flows through aradially extending passage 366 formed at the land 362 to a pressurechamber 368.

While control fluid pressure is being directed from the conduit 114 topressure chambers of the secondary motors in the pump actuatorassemblies 50 and 52 and to pressure chambers of the secondary motors inthe motor actuator assemblies 56 and 58, other chambers in the secondarymotors are connected with drain through the conduit 116. The conduit 116 is connected with drain through an annular passage between lands 364and 374 on the valve spool 350 and the drain passage 358. It should benoted that a pressure chamber 378 is also connected with drain by aradially extending passage 380 extending through the circular land 364(FIG. 6).

As the secondary motors in the pump actuator assemblies 50 and 52 areoperated against the influence of their biasing springs, similar to thebiasing spring 148, the pressure in the conduit 114 increases. Assumingthat the actuator handle 138 was moved in the forward direction througha sufflcient distance, the pressure in the conduit 114 continues toincrease during actuation of the secondary motors in the motor actuatorassemblies 56 and 58 against the influence of spring assemblies similarto the spring assembly 230. The increase in fluid pressure in theconduit 114 is transmitted through the passage 366 in the valve spool350 to the pressure chamber 368 which is located between a pistonelement 382 and the valve spool.

As the pressure in the chamber 368 increases, the valve spool 350 slidesaxially along a support rod 386 from the open position of FIG. 6 towarda closed or centered position against the influence of a biasing spring390. When the valve spool 350 reaches the closed position, the land 362is midway of an annular recess 394 in the housing 354. The passage 114is then connected by the recess 394 with both the control fluid supplyconduit 356 and an entrance 398 to the drain conduit 358. The land 362on the closed or centered valve spool 350 substantially blocks fluidflow to and from the conduit 114 by providing a modulating actionbetween the conduit 114, the control fluid supply conduit 356, and thedrain conduit 358. This modulating action maintains the secondary motorsin the pump actuator assemblies 50 and 52 and in the motor actuatorassemblies 56 and 58 in an operated position which corresponds to theextent to which the control lever 138 is moved from the initial positionshown in dashed lines in FIG. 6. However, when the valve spool 350 is inthe closed position the conduit 114 and the secondary motors of the pumpand motor actuator assemblies 50, 52, 56 and 58 are connected in fluidcommunication with both the drain conduit 358 and the con trol fluidsupply conduit 356 to facilitate rapid response of the secondary motorsto operation of the control valve 42.

Movement of the control handle 138 from the neutral position shown indashed lines in FIG. 3 results in operation of the secondary motors inthe pump actuator assemblies 50 and 52 to an extent which is related tothe distance to which the handle 138 is moved. However, if the handle138 is moved through such a large distance that the pump units 28 and 30reach the fully swashed position, the secondary motors in the motoractuator assemblies 56 and 58 are actuated to decrease the displacementof the motor units 32 and 34. Thus, when the actuator handle 138 ismoved through a relatively large distance, the combined extent to whichthe secondary motors 126 and 212 in the pump actuator assembly 50 andmotor actuator assembly 56 are operated is related to the distance whichthe handle is moved. Similarly, the combined extent to which thesecondary motors in the pump actuator assembly 52 and motor actuatorassembly 58 are operated is related to the distance to which the handle'138 is moved. This results from the fact that the pistons in the varioussecondary motors are moved against the influence of associated biasingsprings and that the valve spool 350 is moved against the influence ofthe biasing spring 390.

The farther the handle 138 is moved in a forward direction from theneutral position, the greater is the extent to which the biasing spring390 is compressed and the greater is the fluid pressure which must bebuilt up in the pressure chamber 368 to move the valve spool 350 fromthe open position of FIG. 6 to the closed position in which the valvespool is centered in the valve.

chamber 352. Of course, the fluid pressure in the chamber 368 isdirectly proportional to the fluid pressure in the conduit 114.Therefore, the greater the fluid pressure in the chamber 368, thegreater the pressure in the motor chambers of the secondary motors forthe pump and motor actuator assemblies 50, 52, 56 and 58. The generalrelationship between the extent to which the handle 138 is moved and theextent to which the control fluid pressure in the conduit 114 is variedis generally similar to that disclosed in U.S. Pat. No. 3,540,220 whichis included herein by this reference thereto.

When the valve spool 350 is in the closed or centered position and animpending stall or overloading condition for the engine 20 isencountered, the anti-stall system 64 will decrease the fluid pressurecommunicated through the conduit 356 to the speed control valve 42. Thedecrease in fluid pressure is communicated to the conduit 114 and thesecondary motors for the pump and motor actuator assemblies 50, 52, 56and 58. The decrease in fluid pressure also results in a decrease in thefluid pressure in the chamber 368 to enable the spring 390 to move thevalve spool 350 to the left from the centered position toward the openposition of FIG. 6. This movement of the valve spool 350 increases thecommunication of the conduit 116 with drain and of the conduit 1 14 withthe fluid pressure supply line 356. Since the fluid pressure in thesupply line 356 has been reduced by the anti-stall system 64, the motoractuator assemblies 56 and 58 return the swashplates of the motor units32 and 34 toward their maximum displace ment conditions. Thereafter, thepump actuator assemblies 50 and 52 move the swashplates of the pumpunits toward their minimum displacement conditions. The valve spool 350remains in the open position until the fluid pressure in the supplyconduit 356 increases sufficiently to again move the valve spool againstthe influence of the biasing spring 234 to the centered or closedposition.

When the control handle is operated in the reverse direction, the valvespool 350 is moved toward the right (as viewed in FIG. 6) to connect theconduit 116 with the fluid supply line 356 and to connect the conduit114 with drain. As the pressure in the conduit 116 builds up, it istransmitted through the radial passage 380 to the pressure chamber 378to move the valve spool to the centered modulating position against theinfluence of the biasing spring 402. Although other known valveconstructions could be utilized, the specific speed control valveconstruction 42 shown in FIG. 6 is preferred due to its cooperation withthe anti-stall system 64. The specific construction of the anti-stallsystem 64 and its interaction with the speed control valve 42 is setforth in the previously mentioned US. Pat. application Ser. No. 237,086filed Mar. 22, 1972 by Edward J. Bojas and entitled HydrostaticTransmission Control System and is incorporated herein in its entiretyby this reference thereto.

Steering Control Valves The input to output speed ratios of the left andrighthand hydrostatic transmission 14 and 16 can be independently variedby operation of the associated one of the steering valves 46 or 48.Thus, the output speed of the left-hand hydrostatic transmission 14 canbe decreased relative to the right-hand hydrostatic transmission 16 byoperating the steering valve 46. IF the steering valve 46 is operatedsufficiently, the left-hand hydrostatic transmission 14 will stopdriving the track 22 (dead track condition) and upon continued operationof the steering valve 46 will reverse the direction of operation of thetrack 22 to enable the vehicle 10 to turn sharply. Of course, operatingthe right-hand steering valve 48 results in a corresponding change inthe operating speed of the right-hand hydrostatic tranmission 16.

When the left-hand steering control valve 46 is in an initial orunactuated position and the main speed control valve 42 is in theforward operated position (as shown in FIG. 6), the steering controlvalve ports high pressure fluid from the conduit 114 tov the conduit 120which is connected in fluid communication with the pressure chamber 142in the secondary motor 126 of the pump actuator assembly 50. Thus, whenthe steering control valve 46 is in its initial position (shown in solidlines in FIG. 6) high pressure fluid from the conduit 114 is conductedthrough an inlet 420 to a cylindrical valve chamber 424 formed in ahousing 426 and enclosing a slidable valve spool 428. Control fluid fromthe inlet 420 passes between annular lands 432 and 434 to an annularrecess 436 connected with the conduit 120 and the pump actuator assembly50.

In addition to connecting the secondary motor 126 in the pump actuatorassembly 50 with control fluid pressure ported from the speed controlvalve 42, the steering control valve 46 connects the conduit 118 and thesecondary motor 126 (FIG. 3) with the speed control valve 42 whichconnects them with drain during forward operation of the hydrostatictransmission 14. The conduit 118 is connected in fluid communicationwith the steering valve chamber 454 through an annular recess 442. Whenthe steering valve 46 is in the unactuated position of FIG. 6, therecess 442 is connected in fluid communication with the conduit 116 by apassage 444 formed in the valve spool 428. The passage 444 has anannular opening 446 which is connected directly with the entrance 442for the conduit 118 and a second annular opening 450 which is connecteddirectly with an opening 454 in the housing 426 when the valve spool 428is in the unactuated position illustrated in FIG. 6. The opening 454 isconnected in fluid communication with the conduit 116 which is connectedwith drain through the speed control valve 42 when the speed controlvalve is in the forward actuated position.

When it is desired to turn the vehicle 10 toward the left (as viewed inFIG. 1) as it is moving forward, the steering valve 46 is actuated todecrease the output speed of the left hand hydrostatic transmission 14relative to the output speed of the right hand hydrostatic transmission16 to thereby decrease the speed of the track 22 relative to the track24. This actuation of the steering valve is effected by pivoting acontrol member or pedal 460 in a counterclockwise direction from theposition shown in solid lines in FIG. 6 toward the position shown indashed lines at 462 in FIG. 6. As the pedal 460 approaches the dashedline position 462, the lands 432 and 434 on the valve spool 428 aremoved into a blocking relationship with the entrances 442 and 436 to theconduits 118 and 120. As the pedal 460 continues to move in acounterclockwise direction from the position illustrated in dashed linesat 462 in FIG. 6 to the position illustrated in dashed lines at 466 inFIG. 6, the entrance 436 to the conduit 120 is connected with theconduit 116 and drain through the speed control valve 42. Similarly, theconduit 118 is connected with the conduit 114 and control fluid pressureby the annular central recess'470 formed between the lands 432 and 434on the valve spool 428. Thus actuation of the steering control valve 46during forward operation of the hydrostatic transmission 14 connects theconduit 120, which was initially connected with control fluid pressure,with drain and connects the conduit 118, which was initially connectedwith drain, with control fluid pressure.

Reversing the drain and control fluid pressure connections for theconduits 118 and 120 results in operation of secondary motor 126 from aforward operating condition toward a reverse operating condition. If theactuator pedal 460 remains in the dashed line position 466 for asufficient length of time, the control fluid pressure conducted to thesecondary motor 126 will reverse the direction of operation of thehydrostatic transmission 14. This is because the piston 144 movesthrough sufficient distance to activate the swashplate motor 132 to movethe swashplate from the forward actuated position of FIG. 4 through theneutral actuated position of FIG. 3 to a reverse actuated position. Ofcourse this effects a reversal in the direction of oper ation of themotor unit 32 to reverse the direction in which the track 22 is drivenby the left hand hydrostatic transmission 14.

If the steering control valve 46 is operated with the motor unit 32 acondition other than the maximum displacement condition of FIG. 4, thecontrol fluid pressure to the secondary motor 212 in the motor actuatorassembly 56 is reduced to effect operation of the motor unit 32 to themaximum displacement condition of FIG. 4 prior to operation of thesecondary motor 126 in the pump actuator assembly 50. This operation ofthe secondary motor 212 in the motor actuator assembly 56 results fromthe fact that the biasing spring assembly 230 in the secondary motor 212has a higher spring load than the biasing spring 148 in the motor 126 ofthe pump actuator assembly 50. Of course, a

continuing reduction of the control fluid pressure results in subsequentoperation of the secondary motor 126 in the pump actuator assembly 50.

if the control pedal 460 for the steering control valve 46 is moved tothe fully actuated position with the swashplate 90 (FIG. 4) of the motorunit 32 in a minimum displacement condition, the motor actuator assembly56 first effects pivotal movement of the motor swashplate 90 back to themaximum displacement condition and then effects pivotal movement of thepump swashplate 70 back to the minimum displacement condition. Duringthe time period in which the swashplate 90 of the motor unit 32 is inits maximum displacement condition, the swashplate 70 in the pump unit28 is moved from its maximum forward displacement condition (shown inFIG. 4) to a maximum reverse displacement condition. When the swashplate90 of the motor unit 32 has returned to the minimum displacementposition with the swashplate 70 of the pump unit 28 in its maximumreverse displacement condition, the speed and direction at which thedrive shaft 36 is driven by the motor unit 32 is reversed to therebyreverse the direction of operation ofthe track 22 relative to the track24 and effect a sharp pivoting or rotation of the vehicle about itscenter.

The steering control valve 48 is of the same construction as thesteering control valve 46 and cooperates with the speed control valve42, pump actuator assembly 52, and motor actuator assembly 58 in thesame manner in which the steering valve 46 cooperates with the speedcontrol valve, motor actuator assembly 50, and pump actuator assembly56. Thus, the steering valve 48 includes a valve chamber which isconnected in fluid communication with the conduits 114 and 116 and withthe secondary motor for the pump actuator assembly 52. Upon operation ofthe steering control valve 48, the displacement of the pump unit can bedecreased and even reversed to effect a slowing down and reversal ofoperation of the motor unit 34. However, it should be noted that whenthe steering valves 46 and 48 are in their unactuated or initialpositions the speed control valve 42 controls the speed and direction ofoperation of the hydrostatic transmissions 14 and 16 to drive the tracks22 and 24 at the same speed in a forward or reverse direction.

When the steering control valve 46 or 48 associated with one ofthehydrostatic transmissions 14 or 16 is actasted. the operation of theother hydrostatic transmission should advantageously be uneffectcd.However, the steering valves 46 and 48 are connected in fluidcommunication with each other and the speed control valve 42 by theconduits 114 and 116. Due to this fluid communication, a high rate offlow of control fluid through one of the steering valves 46 or 48 willtend to reduce the control fluid pressure communicated to the othersteering valve. Of course, reducing the control fluid pressurecommunicated to an unactuated steering valve may result in a operationof the associated pump or motor actuator assembly and a reduction inoutput speed. Therefore, it is necessary to retard the flow of controlfluid from the speed control valve 42 through an actuated steeringcontrol valve 46,0r 48 in order to prevent an undesired reduction in thecontrol fluid pressure conducted to the other steering control valve. Itis also desirable to provide for a relatively unrestricted flow of fluidfrom the secondary motors of the pump and motor actuator assemblies 50,52, 56 and 58 through the steering valves 46 and 48 to the speed controlvalve 42 and drain to provide for an accurate response of the secondarycontrol motors to a change in control fluid pressure.

To provide for a restricted flow of control fluid through the steeringcontrol valves 46 and 48 to the associated pump and motor actuatorassemblies and to enable control fluid to flow freely from theassociated pump and motor actuator assemblies through the steeringcontrol valves to the speed control valve 42 during operation of thehydrostatic transmissions 14 and 16 at either the forward or reversedirection, a pair of orifice and check valve assemblies 480 and 482 (seeFIG. 6) are associated with each of the steering valves 46 and 48. Theorifice and check valve assemblies 480 and 482 restrict fluid flow fromthe speed control valve 42 through the associated steering control valveand enable fluid to flow freely from the associated steering controlvalve to the speed control valve. Thus during forward operation of thehydrostatic transmission 14, the orifice and check valve assembly 480restricts fluid flow from the conduit 114 through the steering valve 46to the conduit which is connected with the pump and motor actuatorassemblies 50 and 56. However during forward operation of thehydrostatic transmission 14, fluid can flow freely from the pump andmotor actuator assemblies 50 and 56 through the orifice and checlcvalveassembly 482 to drain through the conduit 116. When the hydrostatictransmission 14 is being operated in the reverse direction, the orificeand check valve assembly 482 restricts fluid flow from the conduit 116to the pump and motor actuator assemblies 50 and 56 while the orificeand check valve assembly 480 enables fluid to flow freely from the pumpand motor actuator assemblies 50 and 56 to the speed control valve 42and drain.

The orifice and check valve assembly 480 includes a valve member 488which is slidably mounted in a chamber 492 in a housing 494. The valvemember 488 is biased to the illustrated closed position under theinfluence of a spring 496. When the valve member 488 is in the closedposition of FIG. 6, fluid under pressure can flow from the conduit 114to the steering valve chamber 424 through an orifice or restriction 502.The orifice 502 restricts the rate at which fluid can flow from theconduit 114 through the steering valve 46 to the pump and motor actuatorassemblies 50 and 56 during operation of the hydrostatic transmission 14in the forward direction to effectively isolate the pump and motoractuator assemblies 52 and 58 for the right hand transmission 16 fromthe effects of operation of either the steering valve 46 or the pump andmotor actuator assemblies 50 and 56 for the left hand transmission 14even though the actuator assemblies for the right and left handtransmissions are connected in fluid communication with each other andthe speed control valve 42 through the conduit 1 14. Therefore, thesteering valve 46 can be actuated to effect a change in the input tooutput speed ratio of the hydrostatic transmission 14 without changingthe input to output speed ratio of the hydrostatic transmission 16.

When the hydrostatic transmission 14 is to be operated in the reversedirection, fluid is exhausted from the pump and motor actuatorassemblies throughthe conduit 120 to the steering valve 46 and throughthe conduit 1 14 to the speed control valve 42 and drain. To enable thepump and motor actuator. assemblies 50 and 56 to respond quickly to achange in control fluid pressure, it is necessary to provide for arelatively unrestricted flow of fluid from the pump and motor actuatorassemblies 50 and 56 to the speed control valve 42 and drain. To providefor this relatively unrestricted flow, the valve member 488 in theorifice and check valve assembly 480 (H6. 6) can be opened against theinfluence of the biasing spring 496 to provide for relatively freecommunication of fluid from the valve chamber 424 through the inlet 420around the valve member 488 and to the conduit 114. When the valvemember 488 is in the open position, it is spaced from a valve seat andfluid can flow from the inlet 420 through any one of a plurality oflongitudinally extending passages formed in the exterior of the valvemember 488 to the conduit 114. Thus during reverse operation of thehydrostatic transmission 14, the orifice and check valve assembly 480enables fluid to flow freely from the steering valve 46 to the speedcontrol valve 42.

During reverse operation of the hydrostatic transmission 14, the orificeand check valve assembly 482 prevents excessive pressure drops fromoccurring in the conduit 116 during operation of the steering valve 46to thereby prevent an undesired change in the input to output speedratio of the hydrostatic transmission 16. Although only the orifice andcheck valve assemblies 480 and 482 associated with the steering valve 46have been illustrated in FIG. 6, it should be understood that orificeand check valve assemblies of the same construction cooperate with thesteering valve 48 in the same manner. The construction and mode ofoperation of steering valves, similar to the steering valves 46 and 48,and the manner in which they cooperate with ori- Free and check valveassemblies, similar to the assemblies 480 and 482, is disclosed andclaimed in copending application Ser. No. 248,948, filed Apr. 28, 1972,by Harold R. Ward on Control Valve. The specific construction of thesteering control valves 46 and 48 and the specific construction of theassociated orifice and check valve assemblies are not per se, part ofthe present invention. In fact, it is contemplated that the steeringcontrol valves 46 and 48 could be constructed as disclosed in theaforementioned Keith et ai. application Ser. No. 158,455 filed June 30,197i.

Main System Pressure Regulation The hydrostatic transmissions 14 and 16each include main system pressure regulator assemblies 520 and 522 (seeFIG. 1) which relieve excessive fluid pressure in either the high or lowpressure conduits of the hydrostatic loop. The main system pressureregulator assembly 520 is connected across the conduits 80 and 86forming the hydrostatic loop between the pump and motor units 28 and 32of the left-hand hydrostatic transmission 14. The main system pressureregulator assembly 522 is connected across conduits 526 and 528 of thehydrostatic loop interconnecting the pump and motor units 30 and 34 ofthe right-hand hydrostatic transmission 16.

During forward operation of the hydrostatic transmissions 14 and 16,high pressure fluid is conducted through the conduits S0 and 526 fromthe pump units 28, 30 to the motor units 32 and 34. if an excessive loadshould be applied to either one of the two hydrostatic transmissions,the pressure in the high pressure fluid conduit 80 or 526 will exceed apredetermined maximum operating pressure and the associated pressureregulator assembly 520 or 522 will interconnect the high and lowpressure conduits of the hydrostatic loop to by-pass the motor tothereby limit the load on the hydrostatic transmission. During forwardoperation of the hydrostatic transmissions l4 and 16 the fluid returnlines, that is, lines 86 and 528, are connected with drain through lowpressure relief valves when a minimum operating pressure is present toenable the two hydrostatic loops to be supplied with makeup fluidconducted from the common charge pump 62 through check valves, similarto the check valves 98 and 100 of FIG. 2.

During reverse operation of the hydrostatic transmissions l4 and 16, theconduits 86 and 528 conduct the high pressure fluid from the pumps 28and 30 to the motors 32 and 34 while the conduits and 526 return lowpressure fluid to the pumps. During reverse operation the pressureregulator assemblies 520 and 522 relieve excessive fluid pressures inthese conduits in the same manner as they do during forward operation ofthe hydrostatic transmissions, In addition, makeup fluid it suppliedfrom the charge pump 62 to the low pressure fluid return lines 80 and526.

The pressure regulator assembly 520 for the lefthand hydrostatictransmission 14 is illustrated in FIG. 7 and includes a high pressureshuttle valve 532 which is connected in fluid communication with theconduits 80 and 86 of the hydrostatic loop 82. Theshuttle valve 532ports fluid pressure in the high pressure conduit, that is, the conduit80, during forward operation of the left-hand hydrostatic transmission14 and the conduit 86during reverse operation of' the hydrostatictransmission to a high pressure relief valve 536. The high pressurerelief valve 536 opens in response to the presence of excessive fluidpressure in the high pressure in the conduit 80 or 86 and ports the highpressure conduit to low or charge pressure relief valve 540 and a lowpressure shuttle valve 542. The charge pressure relief valve 540 isbiased closed and opens upon exposure to fluid pressure of apredetermined magnitude to port the fluid to drain through a conduit 546to relieve the load on the pump and motor units 28 and 32. The lowpressure shuttle valve 542 connects the high pressure conduit, that is,the conduit 80, during forward operation of the hydrostatic transmission14 and the conduit 86 during reverse operation of the hydrostatictransmission, with the low pressure conduit of the hydrostatic loop tothereby short circuit the hydrostatic loop so that the fluid by-passesthe motor. This relieves the load on the pump and motor units 28 and 32.

The high pressure shuttle valve 532 includes a housing 550 which isconnected to fluid communication with the conduits 80 and 86 of thehydrostatic loop 82 by conduits 552 and 554. A shuttle type valve member558 is disposed in a valve chamber 562 in the housing 550 and is urgedtoward the low pressure one of the fluid conduits 80 or 86 under theinfluence of fluid pressure from the high pressure conduit. Thus, duringforward operation of the hydrostatic transmission 14, the shuttle valvemember 558 is moved to the position shown in FIG. 7 under the influenceof relatively high fluid pressure conducted from the conduit 80 throughthe conduit 552 to the valve chamber 562 to thereby connect the highpressure conduit 80 in fluid communi cation with the high pressurerelief valve 536 through a conduit 566. During reverse operation of thehydrostatic transmission 14, the shuttle valve member 558 moves to theright-hand end of the valve chamber 562 under the influence of fluidpressure conducted from the relatively high pressure conduit 86 throughthe conduit 554. The high pressure conduit 86 is then connected in fluidcommunication with the high pressure relief valve 536 through theconduit 566.

The high pressure relief valve 536 includes a valve member 570 which isdisposed in a valve chamber 574 in a housing 576. A weak biasing spring578 urges the valve member 570 toward a closed position in which itblocks fluid flow from the conduit 566 through a conduit 582 to theshuttle valve 542 and charge pressure relief valve 540. An opening ororifice 586 is provided in a head end 588 of the valve member 570 toenable fluid pressure in the conduit 566 to be communicated from anouter face surface 592 of the valve member 570 to an inner face surface594 of the valve member 570. Since the conduit 566 is connected in fluidcommunication with the high pressure conduit of a hydrostatic loop 82,that is, the conduit 80, when the hydrostatic transmission is beingoperated in a forward direction, the fluid pressure force applied to theinner of back side of the valve member 570 varies as a direct functionof variations of the fluid pressure in the high pressure conduit of thehydrostatic loop. It should be noted that the valve during operation islocated off its seat 596 and the fluid pressures on the opposite sidesof the valve member 570 act on equal areas.

The pressure regulator assembly 522 for the righthand hydrostatictransmission 16 is of the same construction as is the pressure regulatorassembly 520 for the left-hand hydrostatic transmission 14. The pressureregulator assembly 522 includes a high pressure shuttle valve 589 whichis connected in fluid communication with the conduits 526 and 528 sothat shuttle valve member 602 is moved to the illustrated position underthe influence of high pressure fluid in the conduit 526 during forwardoperation of the hydrostatic transmission 16 and is moved to theright-hand end of its valve chamber under influence of high pressurefluid from the conduit 528 during reverse operation of the hydrostatictransmission 16. A high pressure relief valve 606 is of the sameconstruction as the high pressure relief valve 536 and is connected withthe high pressure shut tle valve by a conduit 610. The high pressurerelief valve 606 includes a valve member 614 which is biased closed by aspring member and has an opening or oriflce 616 from the high pressureconduit of the hydrostatic loop for the right-hand transmission 16 sothat pressure is communicated to both sides thereof. Therefore, thefluid pressure acting on the chamber containing the biasing springvaries as a function of variations in the fluid pressure conduit of thehydrostatic loop for the transmission 16. A charge pressure relief valve620 and a low pressure shuttle valve 624 are connected in fluidcommunication with the high pressure relief valve 606 by a fluid conduit628. Upon opening ofthe charge pressure relief valve 620, fluid pressurein the conduit 628 is ported to drain through a conduit 632.

The two identical high pressure relief valves 536 and 606 for thehydrostatic loops of the transmissions l4 and 16 are connected in fluidcommunication with a common pilot relief or control valve 638 by fluidconduits 642 and 644. The pilot relief valve 638 includes a controlvalve 650 which is operable to enable either one or both of the highpressure relief valves 536 and 606 to open when the pressure in the highpressure conduit of the hydrostatic loop exceeds a predetermined maximumpressure.

The control valve 650 has a face surface 654 which is exposed to thefluid pressure in a pressure chamber 656. The pressure chamber 656 isconnected in fluid communication with the high pressure relief valves536 and 606 through check valves 660 and 662 and the conduits 642 and644, respectively. When the pressure in the valve chamber for one of thehigh pressure relief valves 536 or 606 exceeds a predetermined minimumpressure to open the associated check valve against the influence of abiasing spring 664, the fluid pressure in the chamber 656 is equal tothe fluid pressure in the high pressure relief valve. For example, thefluid pressure in the valve chamber 574 of the high pressure reliefvalve 536 is communicated through the conduit 642 and check valve 660 tothe pressure chamber 656.

When the pressure against one of the high pressure relief valves 536 or606 and the fluid pressure in the high pressure conduit of theassociated hydrostatic 100p exceeds a predetermined operating pressure,the control valve 650 is opened against the influence of a biasingspring 670 to enable fluid to be conducted from the pressure chamber 656to a relief chamber 672 and a conduit 674 leading to drain. Thus, whenthe fluid pressure in the chamber 656 exceeds a predetermined pressuredetermined by the biasing force applied against the control valve 650 bythe relatively stiff spring 670, the control valve 650 is opened toconnect the pressure chamber 656 in communication with the reliefchamber 672. Of course, this results in a reduction in the pressure inthe chamber 656 to thereby relieve the pressure applied against at leastone of the high pressure relief valves 536 and 606.

When the pressure against the relief valve 536 is reduced due tooperation of the control valve 650 to its open position, the fluidpressure in the hydrostatic loop 82 is relieved by the low pressureshuttle valve 542 which by-passes fluid so that it does not flow throughthe motor. Upon opening of the high pressure relief valve 536, fluidunder pressure flows from the conduit 566 through the high pressurerelief valve to the conduit 582 and the charge pressure relief valve540. The charge pressure relief valve 540 includes a valve member 680which is urged to a closed position under the influence of a biasingspring 682. The fluid pressure in the conduit 582 if sufficient operatesthe valve member 680 to its open position to connect the conduit 582with drain through the conduit 546 to thereby relieve the pressure inthe high pressure conduit of the hydrostatic loop 82. In addition, thelow pressure shuttle valve 542 connects the high pressure conduit of thehydrostatic loop 32 in fluid communication with the low pressure conduitof the hydrostatic loop to thereby short-circuit the hydrostatic loopand further relieve the load applied to the hydrostatic transmission 14.

During forward operation of the hydrostatic transmission 14, a shuttlevalve member 686 in the low pressure shuttle valve 542 is biased to theposition shown in FIG. 7 against the influence of a biasing spring 688by fluid pressure conducted from the high pressure conduit to a valvechamber 692 by a conduit 694. Fluid pressure against the right-hand end(as viewed in FIG. 7) of the shuttle valve member 686 moves the valvemember to connect the conduit 582 in fluid communication with a conduit698 which is connected in fluid communication with the low pressureconduit 86 of the hydrostatic loop 82 by a conduit 702. Thus, when thefluid pressure in the high pressure conduit of the hydrostatic loop 82exceeds a predetermined maximum operating pressure, the master reliefvalve 638 relieves the pressure applied against the high pressure reliefvalve member 570 and the high pressure relief valve 536 opens to connectthe high pressure conduit 80 with the low pressure conduit 86 of thehydrostatic loop through the low pressure shuttle valve 542.

During operation of the hydrostatic transmission 14 in the reversedirection, the pressure regulator assembly 520 operates in a similarmanner to relieve excessive pressures in the high pressure conduit 86 ofthe hydrostatic loop 82. During reverse operation, the valve member 558of the high pressure shuttle valve 532 is moved toward the right fromthe position shown in FIG. 7 to connect the conduit 86 in fluidcommunication with the relief valve 536. If the pressure in the'conduit86 should exceed a predetermined maximum operating pressure, the masterrelief valve 650 is operated to relieve the pressure applied against thevalve member 570 so that it moves to the open position to connect theconduit 86 in communication with the charge pressure relief valve 540and the low pressure shuttle valve 542 through the conduit 582. The highpressure conduit 86 is connected in fluid communication with the lowpressure conduit 80 through the low pressure shuttle valve 542, sincethe valve member 686 will have moved to the right of the position shownin FIG. 7 to connect the conduit 582 with the conduit 80 through aconduit 706. g

If one of the tracks 22 or 24 of the vehicle 10 encounters a relativelylarge load during operation of the vehicle in a forward direction whilethe other track 24 is subjected to a relatively small load, only thehigh pressure relief valve for the hydrostatic transmission subjected tothe large load will be opened. Thus, if the track 22 (FIG. 1) should besubjected to one excessive operating load while the track 24 issubjected to normal loading, only the high pressure relief valve 536 isopened to relieve the load on the transmission 14. The hydrostatictransmission 16 will continue to operate in the forward direction at thesame speed and will be unaffected by the relieving by the excessivepressure in the hydrostatic loop 82 for the hydrostatic transmission 14.This is because the check valve member 662 is held closed under theinfluence of the fluid pressure in the chamber 656 and prevents fluidpressure applied against the valve member 614 of the high pressurerelief valve 606 from being relieved even though the control valve 650opens under the influence of the biasing pressure applied against thevalve member 570 of the high pressure relief valve 536. The controlvalve 650 closes under the influence of the spring 670. This will resultin a build up in the fluid pressure in the chamber 656 and a restorationof the pressures on the opposite order of the valve member 570 so thatthe high pressure relief valve 536 closes. It should be noted thatduring this time the relief valve 606 remains closed if the fluidpressure in the high pressure conduit of the hydrostatic loop for thetransmission 16 does not exceed normal operating pressures.

During normal operation of the hydrostatic transmissions 14 and 16 it isnecessary to cool the fluid being conducted through the hydrostaticloops. This is accomplished by draining fluid from the hydrostatic loopsand replacing it with fluid which is pumped from a reservoir or coolingtank by the charge pump 62 to the low pressure conduits of thehydrostatic loops for the two transmissions 14 and 16. Thus, duringforward operation of the hydrostatic transmission 14, fluid underpressure from the charge pump 62 is conducted from the conduits 108 (seeFIG. 2), 106, 104 to the check valve 98 which is connected with the lowpressure conduit 86 of the hydrostatic loop 82. As relatively cool fluidfrom the charge pump enters the hydrostatic loop through the check valve98, relatively hot fluid is conducted from the low pressure conduit ofthe hydrostatic loop 82 through the low pressure shuttle valve 542 andthe charge pressure relief valve 540. The biasing force applied againstthe charge pressure relief valve member 680 by the spring 682 isrelatively low so that the fluid pressure in the low pressure conduit ofthe hydrostatic loop 82 opens the charge pressure relief valve 540 toenable fluid to flow through the conduit 546 to drain. The conduit 104is connected with the hydrostatic loop for transmission 16 throughsuitable check valves, similar to the check valves 98 and 100. The lowpressure relief valve 620 enables hot fluid to flow from thetransmission 16 to drain.

Although the pressure regulator assembly 522 for the hydrostatictransmission 16 has not been described in detail, it should beunderstood that the pressure regulator assembly 522 is of the sameconstruction as the pressure regulator assembly 520 and operates in thesame manner. Thus, the pressure regulator assembly 522 cooperates withthe master relief valve assembly 638 to relieve excessive pressures inthe hydrostatic loop of the transmission 16 in response to theapplication of excessive biasing pressures against the valve member 614of the high pressure relief valve 606. In addition, the low pressureshuttle valve 624 and charge pump pressure relief valve 620 cooperatewith the low pressure conduit of the hydrostatic loop the transmission16 to enable relatively hot fluid to be conducted from the low pressureconduit to drain and a relatively cool make up fluid to be supplied tothe hydrostatic loop.

Brake Assembly Upon operation of a parking brake 710 (FIG. 7) to anengaged condition, the fluid pressure required to open the control valve650 in the master relief valve 638 is reduced to prevent loading of thehydrostatic transmissions 14 and 16 against the influence of the brake.The parking brake 710 is operated from a disengaged condition to anengaged condition by moving a control handle 712 is a counterclockwisedirection from the position shown in solid lines to the position shownin dashed lines in FIG. 7. This movement of the control handle 712pivots an eccentric 714 to lower a valve cylinder 716 so that an opening718 in the side wall of the valve cylinder 716 is in alignment with anopening to a conduit 720 leading to an operating cylinder 722 for theparking brake 710. When the opening 718 in the valve cylinder 716 is inalignment with the opening to the conduit 720, the conduit 720 isconnected withdrain through the conduit 674 to reduce the fluid pressurein a pressure chamber 724 in the operating cylinder 722. This enables abiasing spring 726 to move a brake band 728 into secure frictionalengagement with a brake shaft 730.

When the valve cylinder 716 is lowered by rotational movement of theeccentric 714 and actuator handle 712, the force applied by the biasingspring 670 against the valve 650 is reduced. This reduction in thebiasing pressure against the control valve 650 enables the valve to openunder the influence of a relatively low fluid pressure in the chamber656. Therefore, the master relief valve 638 and high pressure reliefvalves 536 and 606 operate to their open conditions whenever the fluidpressure in one of the hydrostatic loops exceeds a predeterminedrelatively low fluid pressure. Of course, this prevents the hydrostatictransmissions 14 and 16 from being subjected to their normal operatingloads when the brake 710 is engaged.

During operation of the vehicle 10, a piston 734 in the operatingcylinder 722 is exposed to fluid pressure conducted from the charge pump62 through a monitor valve 734a, a conduit 736 and an orifice 738. Theorifree 738 prevents an excessive drain on the charge pump 62 when thecontrol handle 712 is operated to connect the conduit 720 with drain.The monitor valve 734a operates to assure that a sufficient pressure isprovided to the system and to the brakes so that the brakes are releasedwhen the valve is opened directing pressure to the system. Since thefluid pressure from the charge pump 62 presses against the spring 726 tomaintain the parking brake 710 in a disengaged condition, if the engine20 should be loaded to such an extent that an impending stall ispresent, they fluid pressure in the conduit 736 is substantially reducedso that the spring 726 can move the piston 734 against the pressure inthe chamber 724 to engage the parking brake 710.

Summary The hydrostatic transmission drive system 12 includes a pair ofvariable displacement hydrostatic transmissions 14 and 16 which aredriven from a common engine 20 to drive tracks 22 and 24 of the vehicle.During relatively low speed operation of the vehicle, the input tooutput speed ratios of the hydrostatic transmissions 14 and 16 aresimultaneously varied by operating a speed control valve 42 to activatepump actuator assemblies 50 and 52 to change the displacement of pumpunits 28 and 30. During high speed oper ation of the vehicle 10,operation of the speed control valve 42 activates motor actuatorassemblies 56 and 58 to vary the displacements of the motors 32 and 34.When it is desired to turn the vehicle 10, one of the steering controlvalves 46 or 48 is actuated to decrease the output speed of theassociated hydrostatic transmission and the speed at which theassociated vehicle track is driven.

When the speed control valve 42 is operated to effect a relatively largeincrease the speed at which the vehicle moves forwwardly in a straightline, high pressure control fluid is conducted through the steeringcontrol valves 46 and 48 to the pump actuator assemblies 50 and 52. Thecontrol fluid activates the pump actuator assemblies 50 and 52 toincrease the displacement of the pump units 28 and 30 to their maximumdisplacements to provide part of the desired increase in vehicle speed.Once this has occurred, relatively high pressure fluid conducted throughselector valves, similar to the selector valve 232, activates the motoractuator assemblies 56 and 58 to decrease the effective displacement ofthe motor vehicle units 32 and 34 to provide the remainder of thedesired increase in vehicle speed.

If the steering control valve associated with one of the hydrostatictransmissions 14 or 16 is operated while the vehicle 10 is movingstraight foward, the motor actuator assembly associated with one of themotor units 32 or 34 is activated to initially increase the effectivedisplacement of the motor unit and thereby decrease the speed at which atrack of the vehicle is driven. The pump actuator assembly for theassociated pump unit 28 and 30 is then activated to decrease theeffective displacement of the associated pump unit 28 or 30 and effect afurther decrease in the speed of one of the vehicle tracks. Continuedactuation of the steering control valve causes the pump actuatorassembly to reverse the displacement of the associated pump unit tothereby reversethe direction of operation of the associated hydrostatictransmission. Once the pump unit has reached its maximum effectivereverse displacement, the motor actuator assembly is activated to againdecrease the displacement of the associated motor unit.

The pump actuator assemblies and 52 are of the same construction andeach include a primary or swashplate motor, similar to the motor 132,which is operable to vary the effective displacement of the associatedpump unit. To effect operation of a swashplate motor, an associatedsecondary motor, similar to the motor 126, is operated to activate apilot valve from a null position, similar to the pilot valve 128, toport fluid under pressure to the swashplate motor. When the swashplatemotor has been operated to an extent corresponding to the extent ofoperation of the associated secondary motor, a floating link feedbackassembly moves the pilot valve back to its null position to stop theswashplate motor. Shuttle type selector valves, similar to the selectorvalve 232, port control fluid pressure to the motor actuator assemblies56 and 58. The motor actuator assemblies 56 and 58 are generally similarin construction to the pump actuator assemblies and each includes asecondary motor which operates a pilot valve to port fluid underpressure to a swashplate motor.

Pressure regulator assemblies 520 and 522 for the hydrostatic loops ofeach of the transmission 14 and 16 are connected with a common masterrelief valve 638. Each of the pressure regulator assemblies 520 and 522includes a high pressure relief valve, similar to the valve 536, whichis operable from a closed condition to a bypass condition to limit thefluid pressure in the hydrostatic loop of the associated transmission.The high pressure relief valve is maintained in a closed condition untilthe master relief valve 638 is opened to reduce the pressure appliedacross the high pressure relief valve spool. The high pressure reliefvalve then opens to connect the high pressure conduit of the hydrostaticloop in the associated hydrostatic transmission with the low pressureconduit of the hydrostatic loop.

It should be understood that although the construction of the pump andmotor actuator assemblies 50 and 52 for the hydrostatic transmission 14has been more fully described than the construction of the pump and-motor actuator assemblies 52 and 58 for the hydrostatic transmission16, the pump actuator assembly 52 has the same construction and mode ofoperation as the pump actuator assembly 50 and the motor actuatorassembly 58 has the same construction and mode of operation as the motoractuator assembly 56. It should also

1. A hydrostatic transmission comprising a pump unit, a motor unit,first conduit means for conducting fluid under pressure from said pumpunit to said motor unit, second conduit means for conducting fluid fromsaid motor unit to said pump unit, said pump unit including a swashplatewhich is movable in one direction from a neutral position to vary therate at which fluid flows through said first conduit means to said motorunit to effect operation of said motor unit in a forward direction andis movable in an opposite direction from the neutral position to causefluid to flow through said second conduit means to said motor unit toeffect operation of said motor unit in the reverse direction, highpressure relief valve means operable between a closed condition blockingfluid flow and an open condition enabling fluid to flow from said firstconduit means to reduce the fluid pressure in said first conduit means,means for applying to said high pressure relief valve means a forcewhich urges said high pressure relief valve means toward the closedcondition and which force varies as a function of variations in thefluid pressure in said first conduit means, first by-pass valve meansfor directing fluid from said first conduit means to said high pressurerelief valve means during operation of said motor means in the forwarddirection and for directing fluid from said second conduit means to saidhigh pressure relief valve means during operation of said motor means inthe reverse direction and second by-pass valve means for directing fluidfrom said high pressure relief valve means to said second conduit meansduring operation of said motor means in the forward direction and fordirecting fluid from said high pressure relief valve means to said firstconduit means during operation of said motor means in the reversedirection, charge pressure relief valve means for enabling fluid to flowfrom said second by-pass valve means to drain during operation of saidmotor unit in either the forward or reverse direction, and control meansfor limiting said force applied to said high pressure relief valve meansso that it does not exceed a predetermined force.
 2. A hydrostatictransmission comprising a variable displacemenT pump unit having amovable swashplate, pump actuator means for varying the displacement ofthe pump unit during both forward and reverse operation of thehydrostatic transmission, said pump actuator means including primarymotor means for moving the swashplate of the pump unit in either one oftwo directions from an initial position, a pilot valve operable betweena null position and any one of a plurality of actuated positionsenabling fluid to flow from a source of fluid to said primary motormeans to effect operation of said primary motor means and movement ofthe swashplate of the pump unit in either one of the two directions,secondary motor means for operating said pilot valve between the nulland actuated positions, said secondary motor means being operable tovarying extents in either one of two directions from an initial positionin response to variations in control fluid pressure conducted to saidsecondary motor means, and feedback means for operating said pilot valvefrom any one of the plurality of actuated positions to the null positionin response to movement of the swashplate of the pump unit by saidprimary motor means to an extent which is a function of the extent ofoperation of said secondary motor means, a motor unit, first conduitmeans for conducting fluid under pressure from said pump unit to saidmotor unit, second conduit means for conducting fluid from said motor tosaid pump unit, high pressure relief valve means operable between aclosed condition blocking fluid flow and an open condition enablingfluid to flow from said first conduit means to reduce the fluid pressurein said first conduit means, means for applying to said high pressurerelief valve means a force which urges said high pressure relief valvemeans toward the closed condition and which force varies as a functionof variations in the fluid pressure in said first conduit means, andcontrol means for limiting said force applied to said high pressurerelief valve means so that it does not exceed a predetermined force. 3.A hydrostatic transmission comprising a variable displacement pump unithaving a movable swashplate, a variable displacement motor unit having amovable swashplate, first conduit means for conducting fluid underpressure from said pump unit to said motor unit, second conduit meansfor conducting fluid from said motor unit to said pump unit, highpressure relief valve means operable between a closed condition blockingfluid flow and an open condition enabling fluid to flow from said firstconduit means to reduce the fluid pressure in said first conduit means,means for applying to said high pressure relief valve means a forcewhich urges said high pressure relief valve means toward the closedcondition and which force varies as a function of variations in thefluid pressure in said first conduit means, control means for limitingsaid force applied to said high pressure relief valve means so that itdoes not exceed a predetermined force, pump actuator means for varyingthe displacement of the pump unit during both forward and reverseoperation of the hydrostatic transmission, said pump actuator meansincluding reversible primary motor means for moving the swashplate ofthe pump unit in either one of two directions from an initial position,a pilot valve operable between a null position and any one of aplurality of actuated positions enabling fluid to flow from a source offluid to said primary motor means to effect operation of said primarymotor means and movement of the swashplate of the pump unit in eitherone of the two directions, reversible secondary motor means foroperating said pilot valve between the null and actuated positions, saidsecondary motor means being operable to varying extents in either one oftwo directions from an initial position in response to variations incontrol fluid pressure conducted to said secondary motor means, andfloating link feedback means for operating said pilot valve from any oneof the plurality of actuated positions to the null Position in responseto movement of the swashplate of the pump unit by said primary motormeans to an extent which is a function of the extent of operation ofsaid secondary motor means, third conduit means for conducting controlfluid pressure to said secondary motor means to effect operation of saidsecondary motor means in a first direction and operation of the pilotvalve to one actuated position porting fluid to said primary motor meansto move the swashplate of the pump unit in a first direction from theinitial position to thereby effect operation of the hydrostatictransmission in the forward direction, fourth conduit means forconducting control fluid pressure to said secondary motor means toeffect operation of said secondary motor means in a second direction andoperation of the pilot valve to another actuated position porting fluidto said primary motor means to move the swashplate of the pump unit in asecond direction from the initial position to thereby effect operationof the hydrostatic transmission in the reverse direction, motor actuatormeans for moving the swashplate of the motor unit through a distancewhich varies as a function of variations in control fluid pressureconducted to said motor actuator means during both forward and reverseoperation of the hydrostatic transmission, fifth fluid conduit means forconducting control fluid pressure to said motor actuator means, selectorvalve means for porting control fluid pressure from said third fluidconduit means to said fifth fluid conduit means during operation of thehydrostatic transmission in the forward direction and for portingcontrol fluid pressure from said fourth fluid conduit means to saidfifth fluid conduit means during operation of the hydrostatictransmission in the reverse direction, and pressure controller meansvarying the control fluid pressure conducted to said third fluid conduitmeans during forward operation of the hydrostatic transmission and tosaid fourth fluid conduit means during reverse operation of thehydrostatic transmission to control the extent to which the swashplatesof the pump and motor units are moved by said pump and motor actuatormeans.
 4. An apparatus comprising a reversible hydrostatic transmissionhaving a variable displacement pump unit with a movable swashplate, saidapparatus comprising pump actuator means for varying the displacement ofthe pump unit during both forward and reverse operation of thehydrostatic transmission by moving the swashplate, said pump actuatormeans including reversible primary motor means for moving the swashplateof the pump unit in either one of two directions from an initialposition, a pilot valve operable between a null position and any one ofa plurality of actuated positions enabling fluid to flow from a sourceof fluid to said primary motor means to effect operation of said primarymotor means and movement of the swashplate of the pump unit in eitherone of the two directions, reversible secondary motor means foroperating said pilot valve between the null and actuated positions, saidsecondary motor means being operable to varying extents in either one oftwo directions from an initial position in response to variations incontrol fluid pressure conducted to said secondary motor means, floatinglink feedback means interconnected between said pilot valve, saidswashplate and said secondary motor means for operating said pilot valvefrom any one of the plurality of actuated positions to the null positionin response to movement of the swashplate of the pump unit by saidprimary motor means to an extent which is a function of the extent ofoperation of said secondary motor means, pressure controller means forvarying the control fluid pressure conducted to said secondary motormeans as a function of the extent of operation of said pressurecontroller means from an initial condition to control the extent towhich the swashplate of the pump unit is moved, a variable displacementmotor unit having a movable swashplate, first conduit means forconducting control fluid pressure to said secondary motor means toeffect operation of said secondary motor means in a first direction andoperation of the pilot valve to one actuated position porting fluid tosaid primary motor means to move the swashplate of the pump unit in afirst direction from the initial position to thereby effect operation ofthe hydrostatic transmission in the forward direction, second conduitmeans for conducting control fluid pressure to said secondary motormeans to effect operation of said secondary motor means in a seconddirection and operation of the pilot valve to another actuated positionporting fluid to said primary motor means to move the swashplate of thepump unit in a second direction from the initial position to therebyeffect operation of the hydrostatic transmission in the reversedirection, motor actuator means for moving the swashplate of the motorunit through a distance which varies as a function of variations incontrol fluid pressure conducted to said motor actuator means duringboth forward and reverse operation of the hydrostatic transmission,third fluid conduit means for conducting control fluid pressure to saidmotor actuator means, and selector valve means for porting control fluidpressure from said first fluid conduit means to said third fluid conduitmeans during operation of the hydrostatic transmission in the forwarddirection and for porting control fluid pressure from said second fluidconduit means to said third fluid conduit means during operation of thehydrostatic transmission in the reverse direction.
 5. An apparatus asdefined in claim 4 wherein said pilot valve in said pump actuator meansis effective in its null position to direct fluid leakage flow to saidreversible primary motor means which is constructed to enable leakage toflow therethrough without actuation thereof.
 6. An apparatus as definedin claim 4 wherein said pilot valve in said pump actuator meanscomprises a valve spool movable in opposite directions from its nullposition, said apparatus further comprising a charge pump for directingcontrol fluid to said pressure controller means and to said pilot valvein said pump actuator means.
 7. An apparatus as set forth in claim 4further including fourth fluid conduit means for conducting controlfluid from said motor actuator means, said selector valve means beingoperable to port control fluid from said fourth fluid conduit means tosaid second fluid conduit means during operation of the hydrostatictransmission in the forward direction and to port control fluid fromsaid fourth fluid conduit means to said first fluid conduit means duringoperation of the hydrostatic transmission in the reverse direction. 8.An apparatus as set forth in claim 4 further including steering valvemeans connected with said first and second fluid conduit meansintermediate said selector valve means and said pressure controllermeans for varying the control fluid pressure conducted to said pumpactuator means and to said motor actuator means while maintaining thesetting of said pressure controller means constant.
 9. An apparatus asset forth in claim 4 wherein said selector valve means includes a valvemember movable between a first position connecting said first fluidconduit means in fluid communication with said third fluid conduit meansand a second position connecting said second fluid conduit means withsaid third fluid conduit means, means for exposing said valve member tothe control fluid pressure in said first fluid conduit means to urgesaid valve member to the first position during operation of thehydrostatic transmission in the forward direction, and means forexposing said valve member to the control fluid pressure in said secondfluid conduit means to urge said valve member to the second positionduring operation of the hydrostatic transmission in the reversedirection.
 10. An apparatus as set forth in claim 4 wherein said motoractuator means includes primary motOr means for moving the swashplate ofthe motor unit from an initial position to an actuated position toreduce the displacement of the motor unit, a motor pilot valve operablebetween a null position and an actuated position enabling fluid to flowto said primary motor means in said motor actuator means, secondarymotor means for operating said pilot valve in said motor actuator meansbetween the open and closed position, said secondary motor means in saidmotor actuator means being connected in fluid communication with saidthird fluid conduit means and being operable to varying extents from aninitial position in response to variations in control fluid pressureconducted through said third fluid conduit means to said motor actuatormeans, and floating link feedback means interconnected between saidhydrostatic motor swashplate, motor pilot valve, and said secondarymotor means for said hydrostatic motor for operating said motor pilotvalve to the null position in response to movement of the swashplate ofthe motor unit to an extent which is a function of the extent ofoperation of said secondary motor means in said motor actuator means.11. An apparatus as set forth in claim 4 wherein the hydrostatictransmission includes a hydrostatic loop for conducting fluid betweenthe pump and motor units, said apparatus further including brake meansoperable from a released condition to an activated condition to retardmovement of a vehicle with which the hydrostatic transmission isassociated, relief valve means operable from a closed position to anopen position in response to the presence of excessive fluid pressure inthe hydrostatic loop, means for urging said relief valve means to theclosed position with a predetermined force, and means for varying thepredetermined force from a relatively large force to a relatively smallforce upon operation of said brake means from the activated condition tothe released condition.
 12. An apparatus as set forth in claim 4 furtherincluding relief valve means operable between a closed condition and anopen condition enabling fluid to flow from a hydrostatic loop in thehydrostatic transmission to thereby limit the fluid pressure in thehydrostatic loop, means for applying to said relief valve means a forcewhich urges said relief valve means toward the closed condition andwhich varies as a function of variations in the fluid pressure in a highpressure portion of the hydrostatic loop, and control means for limitingthe force applied to said relief valve means, said relief valve meansincluding a valve member movable between a closed position blockingfluid flow from the hydrostatic loop to an open position enabling fluidto flow from the hydrostatic loop, said means for applying a force tosaid relief valve means including means for exposing a surface of saidvalve member to the fluid pressure in the high pressure portion of thehydrostatic loop, and said control means including means for exposingsaid surface of said valve member to a source of reduced pressure.
 13. Ahydrostatic transmission comprising a pump unit, a motor unit, firstconduit means for conducting fluid under pressure from said pump unit tosaid motor unit, second conduit means for conducting fluid from saidmotor unit to said pump unit, high pressure relief valve means operablebetween a closed condition blocking fluid flow and an open conditionenabling fluid to flow from said first conduit means to reduce the fluidpressure in said first conduit means, means for applying to said highpressure relief valve means a force which urges said high pressurerelief valve means toward the closed condition and which force varies asa function of variations in the fluid pressure in said first conduitmeans, brake means operable between a release condition and an activatedcondition in which said brake means retards movement of a vehicleassociated with said hydrostatic transmission, said brake meansincluding a brake unit which is separate from said pump and motor Unitsof said hydrostatic transmission, and control means for limiting saidforce applied to said high pressure relief valve means so that it doesnot exceed a predetermined force, said control means including means forvarying the predetermined force from a relatively large predeterminedforce to a relatively small predetermined force in response to operationof said brake means from the release condition to the activatedcondition.
 14. A hydrostatic transmission as set forth in claim 13further including third conduit means for conducting fluid from saidfirst conduit means to said second conduit means, said relief valvemeans includes a valve member movable between a closed position blockingfluid flow through said third conduit means and an open positionenabling fluid to flow through said third conduit means, said means forapplying said force to said relief valve means including means forexposing a surface of said valve member to the fluid pressure in saidfirst conduit means, and said control means including means for exposingsaid surface of said valve member to a source of reduced pressure whensaid force exceeds the predetermined force.
 15. A hydrostatictransmission as set forth in claim 13 wherein said pump unit includes aswashplate which is movable in one direction from a neutral position tovary the rate at which fluid flows through said first conduit means tosaid motor unit to effect operation of said motor unit in a forwarddirection and is movable in an opposite direction from the neutralposition to cause fluid to flow through said second conduit means tosaid motor unit to effect operation of said motor unit in the reversedirection, said hydrostatic transmission including by-pass valve meansfor directing fluid from said first conduit means to said relief valvemeans during operation of said motor means in the forward direction andfor directing fluid from said second conduit means to said relief valvemeans during operation of said motor means in the reverse direction. 16.A hydrostatic transmission as set forth in claim 13 wherein said pumpunit includes a swashplate which is movable in one direction from aneutral position to vary the rate at which fluid flows through saidfirst conduit means to said motor unit to effect operation of said motorunit in a forward direction and is movable in an opposite direction fromthe neutral position to cause fluid to flow through said second conduitmeans to said motor unit to effect operation of said motor unit in thereverse direction, said hydrostatic transmission further including firstby-pass valve means for directing fluid from said first conduit means tosaid high pressure relief valve means during operation of said motormeans in the forward direction and for directing fluid from said secondconduit means to said high pesssure relief valve means during operationof said motor means in the reverse direction and second by-pass valvemeans for directing fluid from said high pressure relief valve means tosaid second conduit means during operation of said motor means in theforward direction and for directing fluid from said high pressure reliefvalve means to said first conduit means during operation of said motormeans in the reverse direction.
 17. An apparatus comprising a reversiblehydrostatic transmission having a variable displacement pump unit with amovable swashplate, fluid pressure responsive pump actuator means forvarying the displacement of the pump unit during both forward andreverse operation of the hydrostatic transmission by moving the pumpswashplate, said pump actuator means including primary motor means formoving the pump swashplate in either one of two directions from aninitial position, a pump pilot valve operable between a null positionand any one of a plurality of actuated positions enabling fluid to flowfrom a source of fluid to said primary motor means to effect operationof said primary motor means and movement of the pump swashplate of thepump unit in either onE of the two directions, secondary motor means foroperating said pump pilot valve between the null and actuated positions,said secondary motor means being operable from an initial position to anextent which varies as a function of variations in control fluidpressure conducted to said secondary motor means, and feedback meansconnected with said pump pilot valve, pump swashplate and secondarymotor means for operating said pump pilot valve from one of theplurality of actuated positions to the null position in response tomovement of the pump swashplate by said primary motor means to an extentwhich is a function of the extent of operation of said secondary motormeans, pressure controller means for varying the control fluid pressureconducted to said secondary motor means during both forward and reverseoperation of the hydrostatic transmission and as a function of theextent of operation of said pressure controller means in one directionfrom an initial condition during forward operation of the hydrostatictransmission and as a function of the extent of operation of saidpressure controller means from the initial condition in anotherdirection opposite from said one direction during reverse operation ofthe hydrostatic transmission to control the extent to which the pumpswashplate is moved, first fluid conduit means for conducting fluidpressure from said pressure controller means to said secondary motormeans, a variable displacement motor unit connected in fluidcommunication with said pump unit and having a movable swashplate, fluidpressure responsive motor actuator means for moving the swashplate ofthe motor unit through a distance which varies as a function ofvariations in control fluid pressure conducted to said motor actuatormeans from said pressure controller means, said motor actuator meansincluding primary motor means for moving the motor swashplate from aninitial position to an actuated position to reduce the displacement ofthe motor unit, a motor pilot valve operable between a null position andan actuated position enabling fluid to flow to said primary motor meansin said motor actuator means, secondary motor means for operating saidmotor pilot valve between the actuated and null positions, and feedbackmeans connected with said motor swashplate, motor pilot valve, and saidsecondary motor means in said motor actuator means for operating saidmotor pilot valve to the null position in response to movement of themotor swashplate to an extent which is a function of the extent ofoperation of said secondary motor means in said motor actuator means,and second fluid conduit means for conducting fluid pressure from saidpressure controller means to said secondary motor means in said motoractuator means during both forward and reverse operation of thehydrostatic transmission.
 18. An apparatus as set forth in claim 17wherein said secondary motor means in said pump actuator means includesmeans defining a first operating chamber for receiving fluid pressurefrom said pressure controller means during operation of said hydrostatictransmission in a forward direction and means defining a secondoperating chamber for receiving fluid pressure from said pressurecontroller means during operation of said hydrostatic transmission in areverse direction, said apparatus further including selector valve meansfor porting fluid pressure from said pressure controller means to saidfirst operating chamber during forward operation of said hydrostatictransmission and for porting fluid pressure from said pressurecontroller means to said second operating chamber during reverseoperation of said hydrostatic transmission, said selector valve meansincluding a valve member movable between a first position porting fluidunder pressure to said first operating chamber and a second positionporting fluid under pressure to said second operating chamber, means forreceiving fluid pressure from said pressure controller means to effectmovement of said valve member to the first positIon upon operation ofsaid pressure controller means in the one direction from the initialcondition, and means for receiving fluid pressure from said pressurecontroller means to effect movement of said valve member to the secondposition upon operation of said pressure controller means in the otherdirection from said initial condition.
 19. An apparatus as set forth inclaim 18 further including steering control valve means connected inseries with said selector valve means and said pressure controller meansfor varying the fluid pressure ported to said first operating chamber bysaid selector valve means during forward operation of said hydrostatictransmission and for varying the fluid pressure ported to said secondoperating chamber during reverse operation of said hydrostatictransmission.
 20. An apparatus as set forth in claim 17 wherein saidsecondary motor means in said pump actuator means includes meansdefining a first operating chamber for receiving fluid under pressurefrom said pressure controller means during operation of said hydrostatictransmission in a forward direction and means defining a secondoperating chamber for receiving fluid under pressure from said pressurecontroller means during operation of said hydrostatic transmission in areverse direction, said apparatus further including selector valve meansfor porting fluid pressure from said pressure controller means to saidfirst operating chamber during forward operation of said hydrostatictransmission, for porting fluid pressure from said pressure controllermeans to said second operating chamber during reverse operation of saidhydrostatic transmission, and for porting fluid pressure from saidpressure controller means to said second conduit means during eitherforward or reverse operation of said hydrostatic transmission.
 21. Anapparatus as set forth in claim 20 further including steering controlvalve means connected in series with said selector valve means and saidpressure controller means for varying the fluid pressure ported to saidfirst operating chamber by said selector valve means during forwardoperation of said hydrostatic transmission, for varying the fluidpressure ported to said second operating chamber during reverseoperation of said hydrostatic transmission, and for varying the fluidpressure ported to said second conduit means during either forward orreverse operation of said hydrostatic transmission.
 22. An apparatus asset forth in claim 17 wherein said feedback means in said pump actuatormeans includes a movable link pivotally connected with said pump pilotvalve, pump swashplate and secondary motor means in said pump actuatormeans, said link being movable in a first direction under the influenceof the pump swashplate to move said pump pilot valve to the nullposition upon movement of the pump swashplate in one of the twodirections to an extent which corresponds to the extent of operation ofsaid secondary motor means in said pump actuator means under theinfluence of fluid pressure conducted thereto from said pressurecontroller means, said link being movable in a second direction underthe influence of said pump swashplate to move said pump pilot valve tothe null position upon movement of the pump swashplate in the other ofthe two directions to an extent which corresponds to the extent ofoperation of said secondary motor means in said pump actuator meansunder the influence of fluid pressure conducted thereto from saidpressure controller means.
 23. An apparatus as set forth in claim 22wherein said feedback means in said motor actuator means includes asecond movable link connected with said motor pilot valve, motorswashplate, and said secondary motor means in said motor actuator means,said second link being movable under the influence of the motorswashplate to move said motor pilot valve to the null position uponmovement of the motor swashplate to an extent which corresponds to theextent of operation of said secondary motor in said motor actuAtor meansunder the influence of fluid pressure conducted thereto from saidpressure controller means.
 24. An apparatus as set forth in claim 17wherein the hydrostatic transmission includes a hydrostatic loop forconducting fluid between said pump and motor units, said apparatusfurther including brake means operable from a released condition to anactivated condition to retard movement of a vehicle with which thehydrostatic transmission is associated independently of the hydrostatictransmission, relief valve means operable from a closed position to anopen position to reduce the fluid pressure in at least a portion of thehydrostatic loop, means for urging said relief valve means to the closedposition under the influence of a fluid pressure force, and means forreducing the fluid pressure force upon operation of said brake meansfrom the activated condition to the released condition.
 25. An apparatusas set forth in claim 17 further including relief valve means operablebetween a closed condition and an open condition enabling fluid to flowfrom a hydrostatic loop in the hydrostatic transmission to thereby limitthe fluid pressure in the hydrostatic loop, means for applying to saidrelief valve means a force which urges said relief valve means towardthe closed condition and which varies as a function of variations in thefluid pressure in a high pressure portion of the hydrostatic loop, andcontrol means for limiting the force applied to said relief valve means,said relief valve means including a valve member movable between aclosed position blocking fluid flow from the hydrostatic loop to an openposition enabling fluid to flow from the hydrostatic loop, said meansfor applying a force to said relief valve means including means forexposing a surface of said valve member to the fluid pressure in thehigh pressure portion of the hydrostatic loop, and said control meansincluding means for exposing said surface of said valve member to asource of reduced pressure.
 26. An apparatus as defined in claim 17wherein said pump pilot valve is effective in its null position todirect fluid leakage flow to said reversible primary motor means whichis constructed to enable leakage to flow therethrough without actuationthereof.
 27. An apparatus comprising a pump unit, a motor unit, firstconduit means for conducting fluid under pressure from said pump unit tosaid motor unit during operation of said pump unit, second conduit meansfor conducting fluid from said motor unit to said pump unit duringoperation of said pump and motor units, relief valve means operablebetween a closed condition blocking fluid flow and an open conditionenabling fluid to flow from said first conduit means to reduce the fluidpressure in said first conduit means, means for applying to said reliefvalve means a fluid pressure force which urges said relief valve meanstoward the closed condition during operation of said pump and motorunits, brake means operable from a released condition to an activatedcondition to retard movement of a vehicle with which said pump and motorunits are associated, said brake means including a brake unit which isseparate from said pump and motor units, and control means for reducingthe fluid pressure force applied to said relief valve means uponoperation of said brake means from said released condition to saidactivated condition.
 28. An apparatus as set forth in claim 27 furtherincluding third conduit means for conducting fluid from said firstconduit means to said conduit means, said relief valve means includes avalve member movable between a closed position blocking fluid flowthrough said third conduit means and an open position enabling fluid toflow through said third conduit means, said means for applying a fluidpressure force to said relief valve means including means for exposing asurface of said valve member to a first fluid pressure, and said controlmeans including means for exposing said surface of said valve member toa source of a secOnd fluid pressure which is less than said first fluidpressure.
 29. An apparatus comprising a pump unit, a motor unit, firstconduit means for conducting fluid under pressure from said pump unit tosaid motor unit, second conduit means for conducting fluid from saidmotor unit to said pump unit, first valve means operable between aclosed condition blocking fluid flow and an open condition enablingfluid to flow from said first conduit means to reduce the fluid pressurein said first conduit means, means for exposing at least a portion ofsaid first valve means to a fluid pressure force urging said first valvemeans toward the closed condition, brake means operable from a releasedcondition to an activated condition to retard movement of a vehicle withwhich said pump and motor units are associated, said brake meansincluding biasing means for effecting operation of said brake means fromthe released condition to the engaged condition and pressure chambermeans for receiving fluid under pressure to hold said brake means in thereleased condition against the influence of said biasing means, secondvalve means operable from a first condition to a second condition toeffect a reduction in the fluid pressure in said pressure chamber meansand operation of said brake means from said released condition to saidactivated condition under the influence of said biasing means and tosimultaneously therewith effect operation of said first valve means fromsaid closed condition to said open condition by effecting a reduction inthe fluid pressure force to which said portion of said first valve meansis exposed, and selectively actuatable control means for effectingoperation of said second valve means from said first condition to saidsecond condition to effect operation of said brake means and a reductionin the fluid pressure in said first conduit means.
 30. An apparatus asset forth in claim 29 wherein said first valve means includes means forconnecting said first conduit means in fluid communication with saidsecond fluid conduit means upon operation of said first valve means tothe open condition.
 31. An apparatus comprising a reversible hydrostatictransmission having a variable displacement pump unit with a movableswashplate, said apparatus comprising fluid pressure responsive pumpactuator means for varying the displacement of the pump unit during bothforward and reverse operation of the hydrostatic transmission by movingthe pump swashplate, said pump actuator means including means defining afirst operating chamber for receiving fluid under pressure duringoperation of said hydrostatic transmission in a forward direction andmeans defining a second operating chamber for receiving fluid underpressure during reverse operation of said hydrostatic transmission,pressure controller means for varying the control fluid pressureconducted to said first and second operating chambers as a function ofthe extent of operation of said pressure controller means from aninitial condition to control the extent to which the pump swashplate ismoved, first fluid conduit means for conducting fluid pressure from saidpressure controller means to said first operating chamber during forwardoperation of said hydrostatic transmission, second fluid conduit meansfor conducting fluid pressure from said pressure controller means tosaid second operating chamber during reverse operation of saidhydrostatic transmission, a variable displacement motor unit connectedin fluid communication with said pump unit and having a movableswashplate, fluid pressure responsive motor actuator means for movingthe swashplate of the motor unit through a distance which varies as afunction of variations in control fluid pressure conducted to said motoractuator means from said pressure controller means, said motor actuatormeans including means defining a third operating chamber for receivingfluid pressure from said pressure controller means during operation ofsaid hydrostatic transmission in the forward and revErse directions,third fluid conduit means for conducting fluid pressure from saidpressure controller means to said third operating chamber in said motoractuator means during forward and reverse operation of said hydrostatictransmission, and selector valve means connected with said first,second, and third conduit means for porting fluid pressure from saidpressure controller means to said first operating chamber during forwardoperation of said hydrostatic transmission, for porting fluid pressurefrom said pressure controller means to said second operating chamberduring reverse operation of said hydrostatic transmission, and forporting fluid pressure from said pressure controller means to said thirdoperating chamber during either forward or reverse operation of saidhydrostatic transmission, said selector valve means including a valvemember movable between a first position porting fluid pressure to saidfirst and third operating chambers and a second position porting fluidpressure to said second and third operating chambers.
 32. An apparatuscomprising a reversible hydrostatic transmission having a variabledisplacement pump unit with a movable swashplate, said apparatuscomprising fluid pressure responsive pump actuator means for varying thedisplacement of the pump unit during both forward and reverse operationof the hydrostatic transmission by moving the pump swashplate, said pumpactuator means including primary motor means for moving the pumpswashplate in either one of two directions from an initial position, apump pilot valve operable between a null position and any one of aplurality of actuated positions enabling fluid to flow from a source offluid to said primary motor means to effect operation of said primarymotor means and movement of the pump swashplate of the pump unit ineither one of the two directions, secondary motor means for operatingsaid pump pilot valve between the null and actuated positions, saidsecondary motor means being operable from an initial position to anextent which varies as a function of variations in control fluidpressure conducted to said secondary motor means, and feedback meansconnected with said pump pilot valve, pump swashplate and secondarymotor means for operating said pump pilot valve from one of theplurality of actuated positions to the null position in response tomovement of the pump swashplate by said primary motor means to an extentwhich is a function of the extent of operation of said secondary motormeans, said pump pilot valve being effective in its null position todirect fluid leakage flow to said reversible primary motor means whichis constructed to enable leakage to flow therethrough without actuationthereof, pressure controller means for varying the control fluidpressure conducted to said secondary motor means to control the extentto which the pump swashplate is moved, first fluid conduit means forconducting fluid pressure from said pressure controller means to saidsecondary motor means, a variable displacement motor unit connected influid communication with said pump unit and having a movable swashplate,pressure responsive motor actuator means for moving the swashplate ofthe motor unit through a distance which varies as a function ofvariations in control fluid pressure conducted to said motor actuatormeans from said pressure controller means, and second fluid conduitmeans for conducting fluid pressure from said pressure controller meansto said motor actuator means.